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):
import numpy as np
S=input()

num=np.zeros(26)
for i in range(len(S)):
    num[int(ord(S[i]))-97]+=1

zero=[i for i, num in enumerate(num) if num==0]
if zero==[]:
    print("None")
else:
    print(chr(zero[0]+97))

Solution (Python):
A = list(input())
A = set(A)

x = 0
for c in 'abcdefghijklmnopqrstuvwxyz':
  if c not in A:
    print(c)
    x = 1
    break
if x == 0:
  print('None')

Solution (Python):
s=input();n=[chr(i) for i in range(97,123) if chr(i) not in s];print(n[0] if n else 'None')

Solution (Python):
S = input()
alpha = ['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 alpha :
    if (i in S) == False :
        print(i)
        exit()
print('None')

Solution (Python):
s = input ()
x = 0
l = ['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 range (len(l)):
  if s.count(l[i]) == 0:
    print (l[i])
    x = 1
    break
if x == 0:
  print ('None')

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

Solution (Python):
S = input()
exist = [False] * 26
char = "abcdefghijklmnopqrstuvwxyz"
for i in range(len(S)):
    for j in range(26):
        if S[i] == char[j]:
            exist[j] = True
            break
for i in range(26):
    if exist[i] == False:
        print(char[i])
        exit()
print("None")

Solution (Python):
import collections

S = list(input())

c = collections.Counter(S)

ans = " "
for i in range(97,123):
    s = chr(i)

    if s in c:
        continue
    else:
        ans = s
        break

print("None" if ans ==" " else ans)

Solution (Python):
import sys
s=list(input())
t=set(s)

r='abcdefghijklmnopqrstuvwxyz'

for i in range(26):
  if r[i] not in t:
    print(r[i])
    sys.exit()
    
print('None')

Solution (Python):
s = input()

import string
a = list(string.ascii_lowercase)

for item in a:
    if item not in s:
        print(item)
        break
else:
    print('None')

Solution (Python):
S = list(input())
Sd = {i:S[i] for i in range(len(S))}
check = {i:0 for i in range(26)}
ans = -1

for k,v in Sd.items():
    check[ord(v)-ord('a')] = 1
    
for k,v in check.items():
    if v == 0:
        ans = k
        break
        
if ans == -1:
    print('None')
else:
    print(chr(ans+ord('a')))

Solution (Python):
S = input()

for i in range(97, 123):
    if chr(i) not in S:
        print(chr(i))
        exit()
print('None')

Solution (Python):
def resolve():
    '''
    code here
    '''
    import string
    abc = string.ascii_lowercase

    S = input()

    
    for item in abc:
        if item not in S:
            print(item)
            break
    else:
        print('None')




if __name__ == "__main__":
    resolve()

Solution (Python):
from collections import defaultdict,deque
import math
def main():
    alph = "abcdefghijklmnopqrstuvwxyz"
    s = input()

    for i in range(26):
        if(s.count(alph[i])==0):
            print(alph[i])
            exit()
    print("None")

if __name__ == '__main__':
    main()

Solution (Python):
alpha_dict = {}
for i,c in enumerate(range(ord('A'),ord('Z')+1)):
  alpha_dict[chr(c).lower()] = 0

s = list(input())
for i in range(len(s)):
  alpha_dict[s[i]] += 1

for k, v in alpha_dict.items():
  if v == 0:
    print(k)
    exit()
print('None')

Solution (Python):
alphabeta = "abcdefghijklmnopqrstuvwxyz"
all_str  = set(alphabeta) 
str = input() 
str = set(str)


use = all_str-str 
if use == set():
  print("None")
else:
  print(sorted(use)[0])

Solution (Python):
L=list(input())
L=list(set(L))
L=sorted(L)
R=["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 range(min(26,len(L))):
  if L[i]!=R[i]:
    print(R[i])
    exit()
if len(L)<26:
  print(R[len(L)])
  exit()
print("None")

Solution (Python):
S = input()
alpha = []

for i in range(97,123):
    alpha.append(chr(i))
    
ans = []
for j in alpha:
    if not j in S:
        ans.append(j)
ans.append(None)

print(ans[0])

Solution (Python):
S = sorted(list(set(input())))
ABC = [chr(i) for i in range(97, 123)]
S_box = ["F" for i in range(26)]
ans = "None"
for i in range(26):
    for j in range(len(S)):
        if S[j] == ABC[i]:
            S_box[i] = ABC[i]
if len(S) == 26:
    pass
else:
    for i in range(26):
        if S_box[i] == "F":
            ans = ABC[i]
            break            
print(ans)

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

if len(set(S)) == 26:
    print("None")
    exit()

S = sorted(list(set(S)))

s = list("abcdefghijklmnopqrstuvwxyz")

def list_difference(list1, list2):
    result = list1.copy()
    for value in list2:
        if value in result:
            result.remove(value)

    return result

result = list_difference(s,S)

print(result[0])

Solution (Python):
s = input()
s = sorted(s)
ascii_lowercase = 'abcdefghijklmnopqrstuvwxyz'
ans = 'None'
for chr_alpha in ascii_lowercase:
  if chr_alpha not in s:
    ans = chr_alpha
    break
print(ans)

Solution (Python):
str1=input()

str2="abcdefghijklmnopqrstuvwxyz"


aaa=[]

for i in str2:
    
    if (i in str1) == False:
        aaa.append(i)

aaa.sort()

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

Solution (Python):
S = input()
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"]
if len(set(S))==26:
    print("None")
else:
    ans = set(ans) ^ set(S)
    print(sorted(ans)[0])

Solution (Python):
a = ["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"]
b = input()
c = []
d = len(b)

for i in range(int(d)):
  c.append(b[i])

e = list(set(c))
f = sorted(e)
g = len(f)
h = int(0)

if g != 26:
  for i in range(int(g)):
    if f[i] == a[i]:
      h = h + 1
    else:
      print(a[i])
      break
else:
  print("None")
  
if h == g:
  print(a[h])

Solution (Python):
s = input()
t = sorted(set(list(s)))
if len(set(s)) >= 26:
    print("None")
    exit()

alp = [chr(i) for i in range(97, 97+26)]
for i in alp:
    if i not in t:
        print(i)
        exit()

Solution (Python):

def resolve():
    import sys
    import string
    input = sys.stdin.readline
    
    
    s = input().rstrip()
    alphabets = set()
    for c in string.ascii_lowercase:
        alphabets.add(c)

    for c in s:
        if c in alphabets:
            alphabets.remove(c)
            if len(alphabets) == 0:
                print("None")
                return

    print(sorted(alphabets)[0])

if __name__ == "__main__":
    resolve()

Solution (Python):
import string

s = list(input())
c = [chr(i) for i in range(97, 97+26)]
count = 0

for i in c:
    if i not in s:
        print(i)
        break
    else:
        count += 1

if count == 26:
    print("None")
else:
    pass

Solution (Python):




s=list(input())
flag='None'
alphabet=list('abcdefghijklmnopqrstuvwxyz')
dic={}
for si in s:
    dic[si]=True

for c in alphabet:
    if dic.get(c,False)==False:
        flag=c
        break
print(flag)

Solution (Python):



import sys
from io import StringIO
import unittest


class TestClass(unittest.TestCase):
    def assertIO(self, input, output):
        stdout, stdin = sys.stdout, sys.stdin
        sys.stdout, sys.stdin = StringIO(), StringIO(input)
        resolve()
        sys.stdout.seek(0)
        out = sys.stdout.read()[:-1]
        sys.stdout, sys.stdin = stdout, stdin
        self.assertEqual(out, output)

    def test_入力例_1(self):
        input = """atcoderregularcontest"""
        output = """b"""
        self.assertIO(input, output)

    def test_入力例_2(self):
        input = """abcdefghijklmnopqrstuvwxyz"""
        output = """None"""
        self.assertIO(input, output)

    def test_入力例_3(self):
        input = """fajsonlslfepbjtsaayxbymeskptcumtwrmkkinjxnnucagfrg"""
        output = """d"""
        self.assertIO(input, output)


def resolve():
    S = input()
    n = [0 for _ in range(26)]
    for i in range(len(S)):
        n[ord(S[i]) - ord("a")] = 1
    for i in range(26):
        if n[i] == 0:
            print(chr(ord("a")+i))
            break
    else:
        print("None")


if __name__ == "__main__":
    
    resolve()

Solution (Python):
s=input()
a=ord("a")
for i in range(26):
  if chr(a+i) in s:
    pass
  else:
    print(chr(a+i))
    break
else:
  print("None")

Solution (Python):
from sys import stdin

s = stdin.readline().strip()
a = set('abcdefghijklmnopqrstuvwxyz')
b = a - set(s)

if len(b) == 0: print('None')
else: print(min(b))

Solution (Python):
def main():
	S = input()
	lst = [False] * 26

	for ch in S:
		lst[ord(ch) - ord("a")] = True

	for i in range(26):
		if lst[i] == False:
			print(chr(i + ord("a")))
			break
	else:
		print("None")

  
if __name__ == "__main__":
  	main()

Solution (Python):
S = set(input())
a = set([chr(i) for i in range(ord('a'), ord('z')+1)])
result = sorted(list(a-S))
if len(result) == 0:
    print('None')
else:
    print(result[0])

Solution (Python):
s = list(input())
s = list(set(s))
s.sort()
if len(s) == 26:
    print("None")
l = ["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 range(26):
    if len(s) <= i:
        print(l[i])
        break
    if s[i] != l[i]:
        print(l[i])
        break

Solution (Python):
ans = []
s = input()
for i in range(26):ans.append(0)
for i in s:
  ans[ord(i)-97] = 1
for i in range(26):
  if(ans[i]==0):print(chr(97+i));exit()
print("None")

Solution (Python):
s = str(input())
moji = 'abcdefghijklmnopqrstuvwxyz'
dic = {}
for i in range(len(moji)):
    dic.setdefault(moji[i],0)
for i in range(len(s)):
    dic[s[i]] += 1
isNone = True
for i in range(len(moji)):
    if dic[moji[i]]==0:
        print(moji[i])
        isNone = False
        break
if isNone:
    print('None')

Solution (Python):
s = list(input())
s = set(s)
m = 'abcdefghijklmnopqrstuvwxyz'
for i in range(len(m)):
    if not m[i] in s:
        print(m[i])
        exit(0)
print('None')

Solution (Python):
S = list(input())
a = [chr(i) for i in range(97, 97 + 26)]

for a in a:
    if S.count(a) == 0:
        print(a)
        break
else:
    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']
s1 = str(input())
s = []
for i in s1:
    s.append(i)
s = list(set(s))
s.sort()
x = 'None'
for h in alp:
    if h not in s:
        x = h
        break
print(x)

Solution (Python):
s1 = list(input())
s2 = list('abcdefghijklmnopqrstuvwxyz')
r = sorted(list(set(s2) - set(s1)))
if len(r) == 0:
  print("None")
else:
  print(r[0])

Solution (Python):

S = list(input())
S = sorted(S)
S = set(S)

A = [chr(o) for o in range(ord('a'),ord('z')+1)]
result = 'None'
for i in A:
    if i in S:
        continue
    else:
        result = i
        break
print(result)

Solution (Python):
S = input()
flag = True

while True:
  if S.count('a') == 0:
    ans = 'a'
    break
  elif S.count('b') == 0:
    ans = 'b'
    break
  elif S.count('c') == 0:
    ans = 'c'
    break
  elif S.count('d') == 0:
    ans = 'd'
    break
  elif S.count('e') == 0:
    ans = 'e'
    break
  elif S.count('f') == 0:
    ans = 'f'
    break 
  elif S.count('g') == 0:
    ans = 'g'
    break
  elif S.count('h') == 0:
    ans = 'h'
    break
  elif S.count('i') == 0:
    ans = 'i'
    break
  elif S.count('j') == 0:
    ans = 'j'
    break
  elif S.count('k') == 0:
    ans = 'k'
    break
  elif S.count('l') == 0:
    ans = 'm'
    break
  elif S.count('n') == 0:
    ans = 'n'
    break
  elif S.count('o') == 0:
    ans = 'o'
    break
  elif S.count('p') == 0:
    ans = 'p'
    break
  elif S.count('q') == 0:
    ans = 'q'
    break
  elif S.count('r') == 0:
    ans = 'r'
    break
  elif S.count('s') == 0:
    ans = 's'
    break
  elif S.count('t') == 0:
    ans = 't'
    break
  elif S.count('u') == 0:
    ans = 'u'
    break
  elif S.count('v') == 0:
    ans = 'v'
    break
  elif S.count('w') == 0:
    ans = 'w'
    break
  elif S.count('x') == 0:
    ans = 'x'
    break
  elif S.count('y') == 0:
    ans = 'y'
    break
  elif S.count('z') == 0:
    ans = 'z'
    break
  else:
    ans = 'None'
    break
print(ans)

Solution (Python):
S = input()
s = [ True ] * 26
for i in range(len(S)):
    s[ord(S[i])-97] = False
for i in range(26):
    if s[i]:
        print(chr(i+97))
        exit()
print(None)

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

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

for i in range(97,123):
    if chr(i) not in S_set:
        print(chr(i))
        exit()
print("None")

Solution (Python):
S = input()

ls = list("abcdefghijklmnopqrstuvwxyz")

ans = "None"

for chara in ls:
  if S.count(chara) == 0:
    ans = chara
    break
print(ans)

Solution (Python):
from string import ascii_lowercase
S=input()
ABC=[i for i in ascii_lowercase]


for i in ABC:
  if not i in S:
    print(i)
    break
else:
  print('None')

Solution (Python):











s = input()

h = {}

alp = "abcdefghijklmnopqrstuvwxyz"

for i in alp:
    h[i] = 0

for i in s:
    h[i] += 1


for k,v in h.items():
    if (v == 0):
        print(k)
        exit()

print("None")

Solution (Python):













s = input()

h = {"a":0,"b":0,"c":0,"d":0,"e":0,"f":0,"g":0,"h":0,"i":0,"j":0,"k":0,"l":0,"m":0,"n":0,"o":0,"p":0,"q":0,"r":0,"s":0,"t":0,"u":0,"v":0,"w":0,"x":0,"y":0,"z":0}

for i in s:
    h[i] += 1


for k,v in h.items():
    if (v == 0):
        print(k)
        exit()

print("None")

Solution (Python):
S = input()
alf = 'abcdefghijklmnopqrstuvwxyz'

s = set()
a = set()

for i in range(len(S)):
  s.add(S[i])
for i in range(len(alf)):
  a.add(alf[i])

str = ''
hoge = a ^ s
for h in hoge:
  str += h
  
str=sorted(str)
if str:
  print(str[0])
else:
  print('None')

Solution (Python):
S=input()
set1=set(S)

for i in range(97,123):
  if chr(i) not in set1:
    print(chr(i))
    exit()
print('None')

Solution (Python):
a = list(input())
done = False
for s in list('abcdefghijklmnopqrstuvwxyz'):
  if a.count(s) == 0 and not done:
    print(s)
    done = True

if not done:
  print("None")

Solution (Python):
S = list(input())
alf = 'abcdefghijklmnopqrstuvwxyz'
ans = 'None'
for i in range(len(alf)):
    if not alf[i] in S:
        ans = alf[i]
        break
print(ans)

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

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

Solution (Python):
S = set(input())
Flag = [True]*26

for i in S:
  Flag[ord(i)-97] = False

for i in range(26):
  if Flag[i]:
    print(chr(97+i))
    exit()

print("None")

Solution (Python):
s = list(input())
c = 0
for i in range(97,97 + 26):
	if chr(i) not in  s:
		print(chr(i))
		break
	c += 1
if c == 26:
  print('None')

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

for i in range(26):
	if chr(97+i) not in s:
		print(chr(97+i))
		exit()

print('None')

Solution (Python):
import string
al = string.ascii_lowercase

S = set(input())

for i in al:
  if i not in S:
    print(i)
    exit()

print("None")

Solution (Python):
S=list(set(input()))
S.sort()
import string
switch=True
A=list(string.ascii_lowercase)
for i in A:
    if not i in S:
        print(i)
        switch=False
        break
if switch:
    print("None")

Solution (Python):
S = input().strip()
for s in "abcdefghijklmnopqrstuvwxyz":
    if not s in S:
        print(s)
        exit()
print("None")

Solution (Python):
import bisect, collections, copy, heapq, itertools, math, string, sys
input = lambda: sys.stdin.readline().rstrip() 
sys.setrecursionlimit(10**7)
INF = float('inf')
def I(): return int(input())
def F(): return float(input())
def SS(): return input()
def LI(): return [int(x) for x in input().split()]
def LI_(): return [int(x)-1 for x in input().split()]
def LF(): return [float(x) for x in input().split()]
def LSS(): return input().split()

def resolve():
    S = SS()

    ans = 'None'
    for i in string.ascii_lowercase:
        if S.count(i) == 0:
            ans = i
            break

    print(ans)

if __name__ == '__main__':
    resolve()

Solution (Python):
import string
S = set(input())
for c in string.ascii_lowercase:
    if c not in S:
        print(c)
        break
else:
    print('None')

Solution (Python):
import sys


inint = lambda: int(sys.stdin.readline())
inintm = lambda: map(int, sys.stdin.readline().split())
inintl = lambda: list(inintm())
instrm = lambda: map(str, sys.stdin.readline().split())
instrl = lambda: list(instrm())

s = input()
sl = []

for i in range(len(s)):
    sl.append(s[i])

for i in range(97, 123):
    if chr(i) not in sl:
        print(chr(i))
        exit()

print("None")

Solution (Python):
al = list("abcdefghijklmnopqrstuvwxyz")

s = list(input())
for i in s:
    if i in al:
        del al[al.index(i)]
        
if len(al) == 0:
    print("None")
else:
    print(al[0])

Solution (Python):
from string import ascii_lowercase

S = input()
string = ascii_lowercase
for s in string:
    if s not in S:
        print(s)
        exit()
print('None')

Solution (Python):
s = input()

a=[0]*200
flag=0
for i in s:
    a[ord(i)]=i
for i in range(ord("a"),ord("z")+1):
    if a[i]==0:
        print(chr(i))
        flag=1
        break
print("None" if flag==0 else "")

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

for a in alp:
    if not a in S:
        print(a)
        break
else:
    print('None')

Solution (Python):
S = input()

chars = set()
all_chars = set([c for c in 'abcdefghijklmnopqrstuvwxyz'])

for c in S:
    chars.add(c)

diff = list(all_chars - chars)
diff = sorted(diff)

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

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):
str1 = input()
li = list(str1)
anli = []
for i in range(26):
    anli.append(0)
for i in range(len(li)):
   anli[ord(li[i]) - 97] += 1
flag = True
mi = 27
for i in range(len(anli)):
    if anli[i] == 0:
        flag = False
        mi = min(i,mi)
if flag:
    print('None')
else:
    print(chr(mi+97))

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):
S = input()

for alph in 'abcdefghijklmnopqrstuvwxyz':
    if alph not in S:
        print(alph)
        exit()

print('None')

Solution (Python):
s=list(input())
n=len(s)
l=list("abcdefghijklmnopqrstuvwxyz")
l.append("None")
for i in range(n):
    if s[i] in l:
        l.remove(s[i])
print(l[0])

Solution (Python):
s = input()
ascii_lowercase = list('abcdefghijklmnopqrstuvwxyz')
s = list(s)
s = set(s)
s = list(s)
s = sorted(s)
n = len(s)
flag = 0

for i in range(n):
    if s[i] != ascii_lowercase[i]:
        print(ascii_lowercase[i])
        flag = 1
        break

if len(s) != len(ascii_lowercase) and flag != 1:
    print(ascii_lowercase[len(s)])
elif flag != 1:
    print('None')

Solution (Python):
S = input()

abc = "abcdefghijklmnopqrstuvwxyz"

dic = {s: True for s in abc}

for s in S:
    if dic[s]:
        dic[s] = False

for d in dic:
    if dic[d]:
        print(d)
        exit()

print("None")

Solution (Python):
import string
s = input()
for c in string.ascii_lowercase:
  if c not in s:
    print(c)
    quit()
print("None")

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

Solution (Python):
t=list((set(list(str(input())))))
t.sort(reverse=False)
al = ('abcdefghijklmnopqrstuvwxyz')
alll=list(al)
for a in alll:
    if a not in t:
        print(a)
        exit()
print('None')

Solution (Python):
import sys
def input(): return sys.stdin.readline().strip()

def main():
    S = input()
    d = set()
    for c in S: d.add(c)
    for i in range(26):
        if chr(i + 97) not in d:
            print(chr(i + 97))
            return
    print("None")
    
    


if __name__ == "__main__":
    main()

Solution (Python):

















import sys, re
from collections import deque, defaultdict, Counter
from math import ceil, sqrt, hypot, factorial, pi, sin, cos, radians
from itertools import accumulate, permutations, combinations, product
from operator import itemgetter, mul
from copy import deepcopy
from string import ascii_lowercase, ascii_uppercase, digits
from bisect import bisect, bisect_left
from fractions import gcd
from heapq import heappush, heappop
from functools import reduce
import numpy as np
def input(): return sys.stdin.readline().strip()
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(): return list(map(int, input().split()))
def ZIP(n): return zip(*(MAP() for _ in range(n)))
sys.setrecursionlimit(10 ** 9)
INF = float('inf')
mod = 10 ** 9 + 7

from string import ascii_lowercase
X = {c for c in ascii_lowercase}
S = input()
for s in S:
    if s in X:
        X.remove(s)
X = sorted(X)
if len(X) == 0:
    print("None")
else:
    print(X[0])

Solution (Python):
S = set(input())
all_ = []
for i in range(97,123):
    all_.append(chr(i))
sa = list(set(all_) - S)
sa.sort()
if sa:
    print(sa[0])
else:
    print('None')

Solution (Python):
S=input()
x="None"
num=[False for i in range(26)]
for i in range(len(S)):
    num[ord(S[i])-ord("a")]=True

for j in range(26):
    if not num[j]:
        x=chr(j+ord("a"))
        break

print(x)

Solution (Python):
S=input()
x="None"
T="abcdefghijklmnopqrstuvwxyz"
num=[0 for i in range(26)]
for i in range(26):
    num[i]+=S.count(T[i])

for j in range(26):
    if num[j]==0:
        x=T[j]
        break

print(x)

Solution (Python):
import sys, re
from math import ceil, floor, sqrt, pi, factorial, gcd
from copy import deepcopy
from collections import Counter, deque
from heapq import heapify, heappop, heappush
from itertools import accumulate, product, combinations, combinations_with_replacement
from bisect import bisect, bisect_left, bisect_right
from functools import reduce
from decimal import Decimal, getcontext

def i_input(): return int(input())
def i_map(): return map(int, input().split())
def i_list(): return list(i_map())
def i_row(N): return [i_input() for _ in range(N)]
def i_row_list(N): return [i_list() for _ in range(N)]
def s_input(): return input()
def s_map(): return input().split()
def s_list(): return list(s_map())
def s_row(N): return [s_input for _ in range(N)]
def s_row_str(N): return [s_list() for _ in range(N)]
def s_row_list(N): return [list(s_input()) for _ in range(N)]
def lcm(a, b): return a * b // gcd(a, b)
sys.setrecursionlimit(10 ** 6)
INF = float('inf')
MOD = 10 ** 9 + 7
num_list = []
str_list = []

def main():
    s = s_input()

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

    for al in alpabet:
        if al not in s:
            ans_list.append(al)

    print(ans_list[0] if len(ans_list) > 0 else 'None')

if __name__ == '__main__':
    main()

Solution (Python):
S = input()
alph="abcdefghijklmnopqrstuvwxyz"
for i in alph:
  if i not in S:
    print(i)
    exit()
print("None")

Solution (Python):
S = input()
dic = {}

for s in S:
  dic[ord(s)] = True
  
for i in range(97, 97+26):
  if i not in dic:
    print(chr(i))
    exit()

print('None')

Solution (Python):
s=set(input())
for i in range(97,97+26):
    c=chr(i)
    if not c in s:
        print(c)
        exit()
print('None')

Solution (Python):
s = list(sorted(set(input())))
lst = [chr(i) for i in range(97, 97+26)]

for i in lst:
    if not(i in s):
        print(i)
        exit()
else:
    print("None")

Solution (Python):
S = input()
a = ['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 a:
        a.remove(i)

print(a[0] if len(a)!=0 else 'None')

Solution (Python):
import string

s = set(input())
alphabets = set(string.ascii_lowercase)

l = sorted(list(alphabets - s))

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

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

result = None

for s_uni in range(ord('a'), ord('z') + 1):
    if not chr(s_uni) in s_list:
        result = chr(s_uni)
        break

if result:
    print(result)
else:
    print('None')

Solution (Python):
S = set(input())
Flag = False
for T in range(26):
    Le = chr(ord('a')+T)
    if Le not in S:
        Flag = True
        break
if Flag:
    print(Le)
else:
    print('None')

Solution (Python):
import sys
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
read = sys.stdin.buffer.read
sys.setrecursionlimit(10 ** 7)
INF = float('inf')

S = input()

set_ = set(S)
for c in [chr(i) for i in range(97, 97+26)]:
    if c not in set_:
        print(c)
        quit()
print('None')

Solution (Python):
CASES = "abcdefghijklmnopqrstuvwxyz"
s=set(input())
for c in CASES:
  if c not in s:
    print(c)
    exit()
print("None")

Solution (Python):
import sys
from string import ascii_lowercase as alphabets
from collections import defaultdict

readline=sys.stdin.readline

s=readline().strip()
d=defaultdict(int)
for x in s:
  d[x]=1
for a in alphabets:
  if d[a]==0:
    print(a)
    sys.exit()
print('None')

Solution (Python):
Sset=set(input())
Sset=sorted(Sset)

ans = "None"
flg = True
last = "None"
for i,c in enumerate(Sset):
	if i+97 !=ord(c):
		flg=False
		ans = chr(i+97)
		break
	last = c

if flg and last != 'z' :
	ans = chr(ord(last)+1)
print(ans)

Solution (Python):
import sys

read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines
sys.setrecursionlimit(10 ** 9)
INF = 1 << 60
MOD = 1000000007


def main():
    S = readline().strip()

    found = [0] * 26
    for c in S:
        found[ord(c) - ord('a')] = True

    ans = 'None'
    for i, f in enumerate(found):
        if not f:
            ans = chr(i + ord('a'))
            break

    print(ans)
    return


if __name__ == '__main__':
    main()

Solution (Python):
import sys
from string import ascii_lowercase

read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines
sys.setrecursionlimit(10 ** 9)
INF = 1 << 60
MOD = 1000000007


def main():
    S = readline().strip()

    s = set(S)
    for c in ascii_lowercase:
        if c not in s:
            print(c)
            return

    print('None')
    return


if __name__ == '__main__':
    main()

Solution (Python):
S=[x for x in input()]
A='abcdefghijklmnopqrstuvwxyz'
for a in A:
  if a not in S:
    print(a)
    break
else:
  print('None')

Solution (Python):
moji = input()
L = len(moji)

lst = []
for i in range(L):
    lst.append(ord(moji[i]))

lst2 = list(range(97,123))
ans = list(set(lst2) - set(lst))

if len(ans) == 0:
    print("None")
else:
    print(chr(min(ans)))

Solution (Python):
d = list(input())
d.sort()

l = list("abcdefghijklmnopqrstuvwxyz")
l.remove(d[0])
for i in range(len(d)-1):
    if d[i+1] != d[i]:
        l.remove(d[i+1])
if len(l) == 0:
    print("None")
else:
    print(l[0])

Solution (Python):
s = input()

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

for i in range(len(s)):
    if s[i] in alphabet:
        alphabet.remove(s[i])

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

Solution (Python):
S = input()
list = [chr(i) for i in range(97, 97 + 26)]
x = 0
for i in range(26):
    if list[i] in S:
        x += 1
if x == 26:
    print('None')
    exit()
for i in range(26):
    if list[i] not in S:
        print(list[i])
        exit()

Solution (Python):
S = list(input())
S.sort()
import bisect
a_uni = ord('a')

Search = S
for i in range(26):
    char = chr(a_uni+i)

    if Search and Search[0] == char:
        ind = bisect.bisect_right(S,char)
        Search = S[ind:]
        continue
    print(char)
    exit()
print('None')

Solution (Python):
S = input()

str_list = list('abcdefghijklmnopqrstuvwxyz')
str_cnt = [0 for i in range(len(str_list))]

for i in range(len(S)):
    s = S[i]
    
    index_s = str_list.index(s)
    str_cnt[index_s] += 1

if (0 not in str_cnt):
    ans = 'None'
else:
    ans = str_list[str_cnt.index(0)]

print(ans)

Solution (Python):
S=set(list(input()))
l=[chr(i+97)for i in range(26)if not chr(i+97)in S]
print(l[0]if len(l)else'None')

Solution (Python):
import collections
S=str(input())
cc=collections.Counter(S)
alphabet=['a','b','c','d','e','f','g','h','i','j','k','l','m','m','o','p','q','r','s','t','u','v','w','x','y','z']

for i in range(26):
    if cc[alphabet[i]]==0:
        print(alphabet[i])
        exit()
    if i==25:
        print('None')
        exit()

Solution (Python):


alpha = {
    '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=set(input())
if len(S) == len(alpha):
    print("None")
else:
    print(sorted(alpha - S)[0])

Solution (Python):
s=input()
S=[]
count=0
for i in range(len(s)):
    S.append(s[i])
for x in range(26):
    if chr(x+ord("a")) in S:
        count=count+1
    else:
        print(chr(x+ord("a")))
        break
if count==26:
    print(None)
else:
    pass

Solution (Python):
S = input()
List = list(S)
s_l = set(List)
n = 10
res= ""
flag = False
for i in range(97,123):
  if not chr(i) in s_l:
    res = chr(i)
    flag = True
    break
if flag:
  print(res)
else:
  print("None")

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

Solution (Python):
S = input()
S = set(S)
for i in range(26):
    w = chr(ord("a") + i)
    if not w in S:
        print(w)
        break
else:
    print("None")

Solution (Python):
s=list(input())
for i in range(ord('a'),ord('z')+1):
    if s.count(chr(i))==0:
        print(chr(i))
        exit()
print('None')

Solution (Python):
abc = "abcdefghijklmnopqrstuvwxyz"
a = input()
for i in range(len(abc)):
    if abc[i] not in a:
        print(abc[i])
        exit()
print("None")

Solution (Python):
s = input()

alphabets = 'abcdefghijklmnopqrstuvwxyz'
for i in s:
  alphabets = alphabets.replace(i, '')
  s = s.replace(i, '')

alphabets = [i for i in alphabets]
alphabets.sort()
if len(alphabets) > 0:
  print(alphabets[0])
else:
  print('None')

Solution (Python):
S = input()

Alpha = "abcdefghijklmnopqrstuvwxyz"
for i in Alpha:
  if i not in S:
    print(i)
    exit()
print("None")

Solution (Python):
S = input()
al = [0] * 26
s = ""

for s in S:
    al[ord(s) - 97] += 1

if 0 not in al:
    print("None")
else:
    print(chr(al.index(0) + 97))