Problem:
 Score : 200 points    Problem Statement You are given a string S consisting of lowercase English letters.
Find the lexicographically (alphabetically) smallest lowercase English letter that does not occur in S .
If every lowercase English letter occurs in S , print None instead.      Constraints  1 \leq |S| \leq 10^5 ( |S| is the length of string S .)  S consists of lowercase English letters.         Input Input is given from Standard Input in the following format:  S       Output Print the lexicographically smallest lowercase English letter that does not occur in S .
If every lowercase English letter occurs in S , print None instead.        Sample Input 1 atcoderregularcontest      Sample Output 1 b  The string atcoderregularcontest contains a , but does not contain b .       Sample Input 2 abcdefghijklmnopqrstuvwxyz      Sample Output 2 None  This string contains every lowercase English letter.       Sample Input 3 fajsonlslfepbjtsaayxbymeskptcumtwrmkkinjxnnucagfrg      Sample Output 3 d

Solution (Python):
s = input()
n = len(s)
alp = [False] * 26
for i in range(n):
    num = ord(s[i]) - ord('a')
    alp[num] = True
ok = 0
for i in range(26):
    if alp[i]:
        continue
    print(chr(ord('a') + i))
    ok = 1
    break
if ok == 0:
    print("None")

Solution (Python):
s = input()
l = [chr(ord('a') + i) for i in range(26)]
for i in l:
    if i not in s:
        print(i)
        exit()
print("None")

Solution (Python):

s=input()
c='a'
while c<='z':
    if c not in s:
        print(c)
        exit()
    c=chr(ord(c)+1)
    
print('None')

Solution (Python):
s = input()
ss = sorted(set(s))
cnt = ord('a')
for c in ss:
    if ord(c) == cnt:
        cnt += 1
    else:
        print(chr(cnt))
        exit()
if cnt == ord('{'):
    print('None')
else:
    print(chr(cnt))

Solution (Python):


alpha = [chr(ord("a") + i) for i in range(26)]
s = input().rstrip()

tmp = [i for i in alpha if i not in s]

if len(tmp) == 0:
    print("None")
else:
    print(tmp[0])

Solution (Python):

S = input()
alp = "abcdefghijklmnopqrstuvwxyz"
for a in alp:
    if a not in S:
        print(a)
        exit()
print("None")

Solution (Python):

alp=["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]

S=input()
lS=list(S)
flg=False

for i in range(len(alp)):
    if alp[i] in lS:
        continue
    else:
        print(alp[i])
        flg=True
        break

if not(flg):
    print("None")

Solution (Python):


al=[chr(ord('a') + i) for i in range(26)]
s = list(input())

for i in s:
    if i in al:
        al.remove(i)

if len(al) == 0:
    print('None')
else:
    print(al[0])

Solution (Python):
letters="abcdefghijklmnopqrstuvwxyz"
ans=sorted(set(letters)^set(input()))
print("None" if len(ans)==0 else ans[0])

Solution (Python):
l = list()
for i in range(97, 123):
    l.append(chr(i))
s = list(set(input()))
for i in s:
    l.remove(i)
if l == []:
    print("None")
else:
    print(l[0])

Solution (Python):
s=sorted(list(set(input())))
al='abcdefghijklmnopqrstuvwxyz'
for i in range(len(s)):
  if s[i]!=al[i]:
    print(al[i])
    exit()
ans=al[len(s)] if len(s)<26 else 'None'
print(ans)

Solution (Python):
alphabet_list = [chr(ord('a')+i) for i in range(26)]
alphabet_string = ''.join(alphabet_list)
s = input()
for i in s:
    alphabet_string = alphabet_string.replace(i, '')
if len(alphabet_string) != 0:
    print(alphabet_string[0])
else:
    print('None')

Solution (Python):
import string

l = list(input())
l.sort()
d = list(string.ascii_lowercase)

if len(set(l)) == 26:
    print('None')
else:
    for i in range(26):
        if d[i] not in l:
            print(d[i])
            break

Solution (Python):
s=input()
alphabet=set([i for i in "abcdefghijklmnopqrstuvwxyz"])
ans=alphabet-set(list(s))
print(sorted(ans)[0]) if len(ans)>0 else print("None")

Solution (Python):
alphabet = 'abcdefghijklmnopqrstuvwxyz'
alphabet_list = list(map(str, alphabet))
s = list(input())
s_set = list(set(s))
for s in s_set:
    if s in alphabet_list:
        alphabet_list.remove(s)
if len(alphabet_list) != 0:
    print(alphabet_list[0])
else:
    print('None')

Solution (Python):
s = input()
l = [False] * 26
for c in s:
  l[ord(c) - ord('a')] = True
for i in range(26):
  if not l[i]:
    print(chr(97+i))
    exit()
print('None')

Solution (Python):
S = input()
alphabet = set([chr(i) for i in range(97, 97+26)])
ans = alphabet - set(list(S))
print(sorted(ans)[0]) if len(ans) > 0 else print('None')

Solution (Python):
s = sorted(list(set(list(input()))))
i = 0
while i < len(s) and chr(i+97)==s[i]:
    i += 1
if i<=25:
    print(chr(i+97))
else:
    print("None")

Solution (Python):
S = str(input())
ans = 'None'
alfabet = 'abcdefghijklmnopqrstuvwxyz'
for i in alfabet:
    if i not in S:
        ans = i
        break
print(ans)

Solution (Python):
N = str(input())
ans = None
for l in "abcdefghijklmnopqrstuvwxyz":
    if l not in N:
        ans = l
        break
print(ans)

Solution (Python):
txt = input()
check_list = [0] * 26
base = ord('a')

for c in list(txt):
    check_list[ord(c) - base] = 1
    
ans = -1    
for i in range(26):
    if check_list[i] == 0:
        ans = base + i
        break
    
if ans == -1:
    print('None')
else:
    print(chr(ans))

Solution (Python):
s = set(list(input()))
a = list("abcdefghijklmnopqrstuvwxyz")

for i in s:
    a.remove(i)

if a:
    print(a[0])
else:
    print("None")

Solution (Python):
S = str(input())

reference = 'abcdefghijklmnopqrstuvwxyz'

for i in range(26):
    if reference[i] not in S:
        print(reference[i])
        break
    elif i == 25:
        print('None')

Solution (Python):
from string import ascii_lowercase


def main():
    string = input()
    count = {}
    for lcase in ascii_lowercase:
        count[lcase] = 0
    for s in string:
        count[s] += 1
    count = sorted(count.items(), key=lambda x: x[0])
    for i, count in enumerate(count):
        if count[1] == 0:
            print(ascii_lowercase[i])
            break
    else:
        print("None")


if __name__ == '__main__':
    main()

Solution (Python):
alphabet='abcdefghijklmnopqrstuvwxyz'
S=input()
ans="None"
for a in alphabet:
    if S.count(a)==0:
        ans=a
        break
print(ans)

Solution (Python):
S = input()

x = "abcdefghijklmnopqrstuvwxyz"
ans = "None"
for i in x:
  if i in S:
    pass
  else:
    ans = i
    break

print(ans)

Solution (Python):
def cal_word(n):
	return n - 97

def cal_word_rev(n):
	return n + 97

S = list(map(str,input()))
S_new = sorted(S)
index = -1
S_ans = []

for i in range(0, 26):
	S_ans.append('false')

for i in range(0, len(S)):
	S_ans[cal_word(ord(S[i]))] = 'true'

for i in range(0, 26):
	if S_ans[i] == 'false':
		index = i
		break
if index == -1:
	print('None')
else:
	print(chr(cal_word_rev(index)))

Solution (Python):
S = input()
eng_list = [i for i in range(97, 123)]
for i in range(len(S)):
    for j in range(len(eng_list)):
        if ord(S[i]) == eng_list[j]:
            eng_list.pop(j)
            break
print(chr(eng_list[0])) if len(eng_list) != 0 else print("None")

Solution (Python):
s = input()
x = "abcdefghijklmnopqrstuvwxyz"
ans = "None"
for i in x:
    if i in s:
        pass
    else:
        ans = i
        break
print(ans)

Solution (Python):
from string import ascii_lowercase as ascii


def main(s):
    _s = set(s)

    for a in ascii:
        if not a in s:
            print(a)
            return

    print('None')


if __name__ == "__main__":
    s = input()

    main(s)

Solution (Python):
A=input()
B=sorted(set(A))

def aaa(a):
  return(ord(a))

C=list(map(aaa,B))

for i in range(26):
  if (i+97 in C)!=1:
    print(chr(i+97))
    break
  if i==25:
    print("None")

Solution (Python):
s = input()
for i in range(97, 123):
    if chr(i) not in s:
        print(chr(i))
        exit(0)
print('None')

Solution (Python):
S = input()
alpha_li = [chr(i) for i in range(97, 97 + 26)]
for s in S :
    if s in alpha_li :
        alpha_li.remove(s)
if alpha_li == [] :
    print("None")
else :
    print(alpha_li[0])

Solution (Python):
S = input()

set_alphabet = {chr(i) for i in range(97,123)}

list_ans = list(set_alphabet - set(S))
list_ans.sort()

if list_ans :
    print(list_ans[0])
else :
    print("None")

Solution (Python):
s=input()
ans="None"
for i in range(97, 97+26):
    if chr(i) not in s:
        ans=chr(i)
        break
print(ans)

Solution (Python):
s = list(input())

A = [chr(i) for i in range(ord("a"), ord("z") + 1)]
t = 0
for i in A:
    if i not in s:
        t += 1
        print(i)
        break
if t == 0:
    print("None")

Solution (Python):
S = input()
atoz = [chr(i) for i in range(ord('a'), ord('z') + 1)]
ans = ''

for s in sorted(atoz, reverse=True):
  if s not in S:
    ans = s

if ans == '':
  print('None')
else:
  print(ans)

Solution (Python):




S = input().strip()

all_alphabet=[chr(ord('a') + i) for i in range(26)]

check_alphabet=all_alphabet
for word in S:
    if word in check_alphabet:
        check_alphabet.remove(word)

if len(check_alphabet) == 0:
    print("None")
else:
    print(check_alphabet[0])

Solution (Python):
diff = set('abcdefghijklmnopqrstuvwxyz') - set(input())
if diff:
    print(sorted(diff)[0])
else:
    print('None')

Solution (Python):
S="abcdefghijklmnopqrstuvwxyz"
T=list(input())
for i in range(26):
  x=S[i]
  if not(x in T):
    print(x)
    break
else:
  print('None')

Solution (Python):
s=str(input())

for i in range(26):
    if s.count(chr(97+i))==0:
        print(chr(97+i))
        break
else:
    print("None")

Solution (Python):
r = list(set(range(97, 123)) - set(map(ord, list(input()))))
if r:
    print(chr(min(r)))
else:
    print('None')

Solution (Python):
S = input()
A = list("abcdefghijklmnopqrstuvwxyz")
flag = [a for a in A if a not in S]

if flag:
    flag.sort()
    print(flag[0])
else:
    print("None")

Solution (Python):
def main():
    S = set(input())
    ans = "None"
    for i in range(97, 97+26):
        if chr(i) not in S:
            ans = chr(i)
            break
    print(ans)

if __name__ == '__main__':
    main()

Solution (Python):
s=list(input())
a=list("abcdefghijklmnopqrstuvwxyz")
a=[i for i in a if i not in s]
if a:
    a.sort()
    print(a[0])
else:
    print("None")

Solution (Python):
S = input()
s = [S[_] for _ in range(len(S))]
s.sort()

judge = "abcdefghijklmnopqrstuvwxyz"
for i in range(len(s)):
  judge = judge.lstrip(s[i])

if judge == "":
  print("None")
else:
  print(judge[0])

Solution (Python):
import sys
lis = list('abcdefghijklmnopqrstuvwxyz')
s = sorted(set(input()))
for i in lis:
    if i not in s:
        print(i)
        sys.exit()
print("None")

Solution (Python):
S = input()

if len(set(S)) == 26:
  print("None")
else:
  for dam in ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]:
    if not dam in list(set(S)):
           break
  print(dam)

Solution (Python):
s = input()
for i in range(26):
    c = chr(ord('a')+i)
    if c not in s:
        print(c)
        exit()
print('None')

Solution (Python):
s=input()
a=list("abcdefghijklmnopqrstuvwxyz")
for x in a:
    if x not in s:
        print(x)
        exit()
print("None")

Solution (Python):
S = input()
alphabet = "abcdefghijklmnopqrstuvwxyz"
A = list(alphabet)
for x in A:
    if x not in S:
        print(x)
        exit()
print("None")

Solution (Python):
S = input()
alp = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']
ans = ''
flg = False
for s in alp:
  if s not in S:
    ans = s
    flg = True
    break
if flg:
  print(ans)
else:
  print('None')

Solution (Python):
s = list(input())
sl = list(int(ord(i)) for i in s)

for i in range(97, 123):
    if sl.count(i) == 0:
        print(chr(i))
        exit()
else:
    print('None')

Solution (Python):
def notfound(s):

    ans = ""
    for code in range(ord('a'), ord('z') + 1):
        if s.count(chr(code)) == 0:
            return chr(code)
    return "None"

def main():
    s = str(input())
    print(notfound(s))

if __name__ == '__main__':
    main()

Solution (Python):
s = sorted(input())
a = [i for i in range(26)]

for i in a:
    if s.count(chr(i + ord("a"))) == 0:
        print(chr(i + ord("a")))
        exit()

print("None")

Solution (Python):
s=list(input())
lis=[]
for i in range(26):
    lis.append(chr(97+i))
for i in range(len(s)):
    if s[i] in lis:
        lis.remove(s[i])
if len(lis)>0: print(lis[0])
else: print('None')

Solution (Python):
s=input()
ans=1
all="abcdefghijklmnopqrstuvwxyz"
for i in all:
    if i not in s:
        print(i)
        ans=0
        break
if ans==1:
    print("None")

Solution (Python):
s = set(input())
for c in 'abcdefghijklmnopqrstuvwxyz':
  if c not in s:
    print(c)
    break
else:
  print('None')

Solution (Python):
str = input()
alpha = [chr(i) for i in range(ord('a'), ord('z')+1)]
ans = "None"
for id in alpha:
    if id not in str: 
        ans = id
        break
print(ans)

Solution (Python):
alpha = [chr(i) for i in range(ord("a"), ord("a")+26)]
s = input()
for i in alpha:
    if i not in s:
        print(i)
        break
    if i == "z":
        print(None)

Solution (Python):
def main():
    s=set(input())
    for a in 'abcdefghijklmnopqrstuvwxyz':
        if a not in(s):
            print(a)
            return
    print('None')

main()

Solution (Python):
l = set(input())
for i in range(26):
    if chr(ord("a")+i) not in l:
        print(chr(ord("a")+i))
        exit()
print("None")

Solution (Python):
alpha = [chr(ord('a') + i) for i in range(26)]

S = set(input())

if len(set(alpha)) == len(S):
    print('None')
else:
    for s in alpha:
        if s not in S:
            print(s)
            exit()

Solution (Python):
s = list(input())
abc = list('abcdefghijklmnopqrstuvwxyz')
abc_exist = [False] * 26

for i in range(len(s)):
  for j in range(26):
    if s[i] == abc[j]:
      abc_exist[j] = True

r = 'None'      
for k in range(26):
  if abc_exist[k] == False:
    r = abc[k]
    break
    

print(r)

Solution (Python):
s = set(input())
abc = set('abcdefghijklmnopqrstuvwxyz')
nots = abc.difference(s)
if len(nots) == 0:
  print('None')
else:
  print(min(nots))

Solution (Python):
try:
    S = input()
    alphabet_lower = [chr(ord('a')+i) for i in range(26)]
    S_ = set(sorted(S))
    ans = set(alphabet_lower)-S_
    try:
        print(sorted(ans)[0])
    except IndexError:
        print('None')
except EOFError:
    pass

Solution (Python):
alphabet = [i for i in "abcdefghijklmnopqrstuvwxyz"]

S = input()
S_list = [i for i in S]
S_list = sorted(set(S_list),reverse=False)

ans = "a"

for index,i in enumerate(S_list):
    if S_list == alphabet:
        ans = None
        break
    else:
        if i != alphabet[index]:
            ans = alphabet[index]
            break
        else:
            ans = alphabet[index+1]

print(ans)

Solution (Python):
S=input()
flag=0
l=[0]*26
for i in range(len(S)):
    if (0<=(ord(S[i])-97))and((ord(S[i])-97)<26):
        l[ord(S[i])-97]=l[ord(S[i])-97]+1
for i in range(26):
    if l[i]==0:
        print(chr(i+97))
        flag=1
        break
if flag==0:
    print("None")

Solution (Python):
from string import ascii_lowercase
s=set(input())
l=sorted([x for x in ascii_lowercase if not x in s])
if l:
  print(l[0])
else:
  print("None")

Solution (Python):
s_in = input()
s = set(s_in)
lst = [ord(i) - ord('a') for i in s]
lst.sort()
for i in range(26):
    if not i in lst:
        print(chr(i + ord('a')))
        break
    elif i == 25:
        print('None')

Solution (Python):
ALPHABET = ["a", "b", "c", "d", "e", "f", "g", "h",
            "i", "j", "k", "l", "m", "n", "o", "p",
            "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]

s = input()
for i in range(26):
    if ALPHABET[i] not in s:
        print(ALPHABET[i])
        break
else:
    print("None")

Solution (Python):
ALPHABET = ["a", "b", "c", "d", "e", "f", "g", "h",
            "i", "j", "k", "l", "m", "n", "o", "p",
            "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]

s = input()
for i in range(26):
    if s.count(ALPHABET[i]) == 0:
        print(ALPHABET[i])
        break
else:
    print("None")

Solution (Python):
ALPHABET = "abcdefghijklmnopqrstuvwxyz"

s = input()
for i in range(26):
    if s.count(ALPHABET[i]) == 0:
        print(ALPHABET[i])
        break
else:
    print("None")

Solution (Python):
ALPHABET = "abcdefghijklmnopqrstuvwxyz"

s = list(input())
for i in range(26):
    if ALPHABET[i] not in s:
        print(ALPHABET[i])
        break
else:
    print("None")

Solution (Python):
ALPHABET = "abcdefghijklmnopqrstuvwxyz"

s = list(input())
for i in ALPHABET:
    if i not in s:
        print(i)
        break
else:
    print("None")

Solution (Python):
ALPHABET = "abcdefghijklmnopqrstuvwxyz"

s = list(input())
s.sort()
for i in ALPHABET:
    if i not in s:
        print(i)
        break
else:
    print("None")

Solution (Python):
ALPHABET = "abcdefghijklmnopqrstuvwxyz"

s = list(input())
s.sort()
for i in ALPHABET:
    if not(i in s):
        print(i)
        break
else:
    print("None")

Solution (Python):
s = input()
a = "abcdefghijklmnopqrstuvwxyz"
for i in a:
    if i not in s:
        print(i)
        break
else:
    print('None')

Solution (Python):
s = input()

dic1 = dict()

for i in s:
    if not i in dic1:
        dic1[i] = 1

for i in range(ord("a"),ord("z")+1):
    if not chr(i) in dic1:
        print(chr(i))
        exit()

print("None")

Solution (Python):
a = "abcdefghijklmnopqrstuvwxyz"
s = set(input())
c = 0

for i in s:
    a = a.replace(i,"")
    c += 1
if c < 26:
    print(a[0])
else:
    print("None")

Solution (Python):


import math


def main():
    S = list(input())
    for abc in 'abcdefghijklmnopqrstuvwxyz':
        if not abc in set(S):
            print(abc)
            exit(0)
    print('None')


if __name__ == "__main__":
    main()

Solution (Python):
strings = input()
alpha = [i for i in range(97, 123)]
srcset = set()
for c in strings:
    srcset.add(ord(c))
for c in srcset:
    alpha.remove(c)
dst = chr(alpha[0]) if alpha else None
print(dst)

Solution (Python):
s = input()
cout = "abcdefghijklmnopqrstuvwxyz"

for i in cout:
    if i not in s:
        print(i)
        break
else:
    print('None')

Solution (Python):
s = input()
alf = 'abcdefghijklmnopqrstuvwxyz'
for c in alf:
  if c not in s:
    print(c)
    break
else:
  print('None')

Solution (Python):
s=input()
alpha=[chr(x) for x in range(ord('a'),ord('z')+1)]

for ss in s:
    if ss in alpha:
        alpha.remove(ss)
        if len(alpha) == 0:
            alpha.append(None)

print(alpha[0])

Solution (Python):
S = input()

for i in range(97,123):
    a = chr(i)
    if a not in S:
        print(a)
        exit()

print("None")

Solution (Python):
s = input()
 
if len(set(s)) == 26:
  print("None")
else:
  for i in range(97, 123):
    if chr(i) not in s:
      print(chr(i))
      break

Solution (Python):
s=set(list(input()))
if len(s)==26:print(None)
else:
    for x in range(97,123):
        if not chr(x) in s:
            print(chr(x))
            break

Solution (Python):
s = input()
u = set()
for c in s:
  u.add(c)

for c in "abcdefghijklmnopqrstuvwxyz-":
  if(c == "-"):
    print("None")
  elif(c not in u):
    print(c)
    break

Solution (Python):
S = input()

S = set(S)
flag = True
alphabet = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
for i in alphabet:
    if i not in S:
        print(i)
        flag = False
        break
if flag:
    print("None")

Solution (Python):
s=input()
ans="None"
for i in range(97,97+26):
	if not chr(i) in s:
		ans=chr(i)
		break
print(ans)

Solution (Python):
S=list(input())
import string
X= list(string.ascii_lowercase)
for i in X:
    if i not in S:
        print(i)
        break
else:
    print("None")

Solution (Python):
s=input()
alphabet = set([chr(i) for i in range(97, 97+26)])
x=set([s[i] for i in range(len(s))])
y=alphabet - x
if len(y)==0:
  print("None")
  exit()
z=list(y)
z.sort()
print(z[0])

Solution (Python):
s = input()
da = 'abcdefghijklmnopqrstuvwxyz'
flag = True
for i in da:
  for j in s:
    if j == i:
      flag = False
  if flag:
    print(i)
    flag = False
    break
  else:
    flag = True
if flag:
  print('None')

Solution (Python):
a=set([chr(i) for i in range(97,97+26)])
l=set(input())
x=sorted(list(a^l))
print(x[0] if len(x)!=0 else "None")

Solution (Python):
s = input()
alph_list = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
res = [0] * 26
for i in range(len(s)):
    for j in range(26):
        if s[i] == alph_list[j]:
            res[j] = 1
for i in range(26):
    if res[i] == 0:
        print(alph_list[i])
        break
    elif i == 25:
        print("None")

Solution (Python):
import string
abc = list(string.ascii_lowercase)

S = list(set(input()))
S.sort()

for i in range(26):
    if abc[i] in S:
        continue
    else:
        print(abc[i])
        exit()
print('None')

Solution (Python):
import string
strings = string.ascii_lowercase
a = input()
ans = "None"
for i in strings:
    if i not in a:
        ans = i
        break

print(ans)

Solution (Python):
s = list(input())
for i in "abcdefghijklmnopqrstuvwxyz":
  if i not in s:
      print(i)
      break
else:
  print("None")

Solution (Python):
ans = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
num = [0] * 26
s = input()
for i in range(len(s)):
	num[ans.index(s[i])]+=1
for i in range(len(num)):
	if num[i] == 0:
		print(ans[i])
		exit()
print("None")

Solution (Python):
S = set(input())
for a in "abcdefghijklmnopqrstuvwxyz":
    if a not in S:
        print(a)
        break
else:
    print("None")

Solution (Python):
import numpy as np

def main():
    alphabet = list("abcdefghijklnopqrstuvwxyz")
    str = input()

    found_chars = sorted(set(str), key=lambda x:ascii(x))

    for chars in found_chars:
        if chars in alphabet:
            del alphabet[alphabet.index(chars)]

    if len(alphabet) == 0:
        print("None")
    else:
        print(alphabet[0])



if __name__ == "__main__":
    main()

Solution (Python):
import math
import sys



s = list(str(input()))








alpha = [chr(i) for i in range(97, 97+26)]
s.sort()
count = 0
ii = 0
n = len(s)
n_alp = len(alpha) - 1
for i in range(n):
  if(s[i] == alpha[ii]):
    ans = alpha[ii]
    ii = ii + 1
    if(ii > n_alp):
      print('None')
      sys.exit()

print(alpha[ii])

Solution (Python):
s = input()
flag = 0
count = 0
for i in range(97,97+26):
    flag = 0
    for j in range(len(s)):
        if chr(i) == s[j]:
            flag = 1
            break
    if flag == 0:
        print(chr(i))
        break
    count += 1

if count == 26:
    print('None')

Solution (Python):
all_c = 'abcdefghijklmnopqrstuvwxyz'
all_key = set(all_c)

S = set(input())

dif = all_key - S

_dif = list(dif)
_dif.sort()

if len(_dif):
    print(_dif[0])
else:
    print('None')

Solution (Python):
"""
author : halo2halo
date : 30, Jan, 2020
"""

import sys




read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
sys.setrecursionlimit(10 ** 7)

S = readline().decode('utf8').rstrip()
m = list('abcdefghijklmnopqrstuvwxyz')

sa = list(S)
sa = list(set(sa))
sa.sort()
ans = ''
for i in m:
    if not (i in sa):
        ans = i
        break

print(ans if ans else 'None')

Solution (Python):
import math
import calendar
import fractions
import itertools

s = input()

a = "abcdefghijklmnopqrstuvwxyz"
flag = 1
for i in a:
    if i not in s:
        flag = 0
        print(i)
        break
if flag == 1:
    print("None")

Solution (Python):
letters="abcdefghijklmnopqrstuvwxyz"
ans=sorted(set(letters)^set(input()))
print("None" if len(ans)==0 else ans[0])

Solution (Python):
s=str(input())
abc=["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
ans = "None"
sort_s = sorted(s)

for i in abc:
    if (i in sort_s) == False:
        ans = i
        break
print(ans)

Solution (Python):
s = str(input())
abc = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
ans = "None"
sort_s = sorted(s)
for i in abc:
  if (i in sort_s)==False:
    ans = i
    break
print(ans)

Solution (Python):
from string import * 
s = input()
a = ascii_lowercase

for i in a:
    if i not in s:
        print(i)
        break
    if i == "z":
        print("None")
        break

Solution (Python):

import sys
s = input()
for i in range(26):
    if chr(97+i) not in s: 
        print(chr(97+i)) 
        break
    if i == 25:
        print("None")

Solution (Python):
INT = lambda: int(input())
INTM = lambda: map(int,input().split())
STRM = lambda: map(str,input().split())
STR = lambda: str(input())
LIST = lambda: list(map(int,input().split()))
LISTS = lambda: list(map(str,input().split()))
def do():
    alp='abcdefghijklmnopqrstuvwxyz'
    s=STR()
    flg=0
    for i in alp:
        if not(i in s):
            print(i)
            flg=1
            break
    if flg==0:
        print('None')
if __name__ == '__main__':
    do()

Solution (Python):
ans=ord("a")
for s in sorted(input()):
    if ord(s)>ans:
        print(chr(ans))
        break
    elif ord(s)==ans:
        ans+=1
else:
    if ans==ord("z")+1:
        print("None")
    else:
        print(chr(ans))

Solution (Python):
S = input()
for i in range(ord("a"), ord("z")+1):
    if chr(i) not in S:
        print(chr(i))
        break
else:
    print("None")

Solution (Python):
import collections
S=sorted(str(input()))

A=[]
L = collections.Counter(S)
values, counts = zip(*L.most_common())
for i in range(len(values)):
  A.append(values[i])
A.sort()

x=0
for i in range(len(A)):
  if ord(A[i])!=i+97:
    x=i+97
    break
if x!=0:
  print(chr(i+97))    
else:
  if len(A)==25:
    print("z")
  else:
    print("None")

Solution (Python):
s = list(set([ord(i) for i in input()]))
for i in range(ord('a'), ord('a')+26):
  if i not in s:
    print(chr(i))
    break
else:
  print('None')

Solution (Python):
S = input()
S = set(list(S))
alp = [chr(ord('a') + i) for i in range(26)]
for al in alp:
    if al in S:
        pass
    else:
        print(al)
        exit()
print("None")

Solution (Python):
import string

S = set(input())
s_list = list(string.ascii_lowercase)

for s in S:
    s_list.remove(s)

if s_list:
    print(s_list[0])
else:
    print('None')

Solution (Python):
import string
letters=string.ascii_lowercase
s=set(input())
print(min((set(letters)^s) or ["None"]))

Solution (Python):
S = input()

t = list(sorted(list(set(chr(i) for i in range(ord('a'), ord('z') + 1)) - set(S))))
if len(t) == 0:
    print('None')
else:
    print(t[0])

Solution (Python):
string=input()
s=set(list(string))

full=set(list("abcdefghijklmnopqrstuvwxyz"))

tmp=full-s
if len(tmp)==0:
  print("None")
  exit()
print(min(tmp))

Solution (Python):
s = input()
alpha_list = "abcdefghijklmnopqrstuvwxyz"
s = set(s)
for letter in alpha_list:
    if letter not in s:
        print(letter)
        break
    else:
        continue
else:
    print("None")

Solution (Python):
S = set(input())
all = 'abcdefghijklmnopqrstuvwxyz'



for i in range(len(all)):
    if not all[i] in S:
        print(all[i])
        exit()

print('None')

Solution (Python):
import sys
s=list(input())
s=list(set(s))
s.sort()
s.append("eof")

for i in range(26):
    if chr(ord("a")+i)!=s[i]:
        print(chr(ord("a")+i))
        sys.exit()
print("None")

Solution (Python):
al=[chr(ord('a')+i) for i in range(26)]
s=input()
for i in al:
  if not(i in s):
    print(i)
    exit()
print('None')

Solution (Python):
from sys import stdin, stdout

s = stdin.readline().strip('\n')
al = [True for i in range(26)]
for i in s:
  al[ord(i) - 97] = False
for i in range(97, 123):
  if al[i - 97]:
    stdout.write(str(chr(i)) + '\n')
    break
else:
  stdout.write('None\n')

Solution (Python):
import string
alphabet = set(list(string.ascii_lowercase))
s = set(input())
rest = sorted(alphabet - s)

ans = rest[0] if rest else 'None'

print(ans)

Solution (Python):
b=input()
for i in range(ord("a"),ord("z")+1):
    if chr(i) not in b:
        print(chr(i))
        exit(0)
print("None")

Solution (Python):
S = set(chr(ord("a")+i) for i in range(26))-set(input())
print(min(S) if S else "None")

Solution (Python):
s = str(input())
alphabet = ["a","b","c","d","e","f"
,"g","h","i","j","k","l","m","n",
"o","p","q","r","s","t","u","v","w","x","y","z"]

for i in s:
    if i in alphabet:
        alphabet.remove(i)
if len(alphabet)!=0:
    print(alphabet[0])
else:
    print("None")

Solution (Python):
a=[chr(i) for i in range(97,97+26)]
s=set(input())
for i in a:
  if not i in s:
    print(i)
    break
else:
  print("None")

Solution (Python):
s=str(input())
alphabet=["a","b","c","d","e","f"
,"g","h","i","j","k","l","m","n",
"o","p","q","r","s","t","u","v","w","x","y","z"]
for i in s:
    if i in alphabet:
        alphabet.remove(i)
if len(alphabet)!=0:
    print(alphabet[0])
else:
    print("None")

Solution (Python):
s = set(input())

alphabet = "abcdefghijklmnopqrstuvwxyz"

for l in alphabet:
  if l not in s:
    print(l)
    break
else:
  print("None")

Solution (Python):
list = 'abcdefghijklmnopqrstuvwxyz'
s = input()

for i in list:
    if i not in s:
        print(i)
        exit()
print('None')

Solution (Python):
S = set(input())

alphabet = [chr(i) for i in range(97,97+26)]

for a in alphabet:
    if a not in S:
        print(a)
        import sys
        sys.exit()

print('None')

Solution (Python):
alpha = list("abcdefghijklmnopqrstuvwxyz")
s = input()

found = False
for chr in s:
    if chr in alpha:
        del alpha[alpha.index(chr)]

if len(alpha) == 0:
    print("None")
else:
    print(alpha[0])

Solution (Python):
lis = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]
n = input()
for s in lis:
    if s in n:
        pass
    else:
        ans = s
        break
    if s == "z":
        ans = "None"
print(ans)

Solution (Python):
sList = sorted(list(set(list(input()))))
chI = ord('a')
for i in sList:
  if chI == ord(i):
    chI += 1
  else:
    print(chr(chI))
    exit(0)
if len(sList) != 26:
  print(chr(chI))
else:
  print("None")

Solution (Python):
s = input()
alpha = [False]*26
ans = "None"

for c in s:
    alpha[ord(c)-ord("a")] = True

for i in range(26):
    if alpha[i] == False:
        ans = chr(ord("a")+i)
        break

print(ans)

Solution (Python):
s = input()
alpha = [chr(ord("a")+i) for i in range(26)]
ans = "None"

for c in alpha:
    if s.count(c) == 0:
        ans = c
        break

print(ans)

Solution (Python):
s = input()
l = len(s)
al = [chr(ord('a') + i) for i in range(26)]
a = 0
ans = "None"
for j in al :
    for i in range(l) :
        if s[i] == j :
            a +=1
    if a == 0 :
        ans = j
        break
    a = 0
print(ans)

Solution (Python):

S = list(input())


alphabets = [chr(ord('a') + i) for i in range(26)]


excludeS = []
flag = True
for i in range(len(alphabets)):
    if alphabets[i] not in S and all(97<=ord(elem)<=122 for elem in S):
        excludeS.append(alphabets[i])
        flag = False

if flag:
    print('None')
else:
    
    sortedExcludeS = sorted(excludeS)
    print(sortedExcludeS[0])

Solution (Python):
S = list(input())

flg = True

for i in range(ord('a'), ord('z') + 1):
    if S.count(chr(i)) == 0:
        flg = False
        ans = chr(i)
        break

print("None" if flg else ans)

Solution (Python):
S =  list(input())
S.sort()
S = set(S)
alphabet = list("abcdefghijklmnopqrstuvwxyz")

for i in alphabet:
    if i in S:
      if i == "z":
        print("None")
      else:
          pass
    elif i not in S:
        print(i)
        break