Buggy python submission (verdict: Time Limit Exceeded):
# import sys
from sys import stdout
from copy import copy, deepcopy
from functools import lru_cache
from string import ascii_lowercase
# from math import inf
inf = float('inf')


def noop(*object): pass
# print = noop # use print() to debug and uncomment this line to mute it

def main():
    ans = solve_case()
    stdout.write("{}\n".format(ans))

def solve_case():
    S = read_str()

    @lru_cache(maxsize=None)
    # the position of the first `char` from `from_idx`
    def next_char_pos(from_idx, char):
        if from_idx == len(S):
            return len(S)
        if S[from_idx] == char:
            return from_idx
        return next_char_pos(from_idx+1, char)

    recon = dict()

    @lru_cache(maxsize=None)
    # the length of the shortest "non subsequence" in S[from_idx:]
    def non_subseq_len(from_idx):
        if from_idx == len(S):
            return 1
        if from_idx > len(S):
            return 0
        ans = inf
        for letter in ascii_lowercase:
            new_ans = non_subseq_len(next_char_pos(from_idx, letter) + 1) + 1
            if ans > new_ans:
                ans = new_ans
                recon[from_idx] = (letter, next_char_pos(from_idx, letter) + 1)
        return ans

    for from_idx in reversed(range(len(S))):
        non_subseq_len(from_idx)

    ans = ''
    idx = 0
    while idx < len(S):
        letter, next_idx = recon[idx]
        ans += letter
        idx = next_idx
    return ans

#################################

def read_str(): return input()
def read_int(): return int(input())
def read_str_list(): return input().split(' ')
def read_int_list(): return list(map(int, input().split(' ')))
def read_lines(n, read_func): return [read_func() for _ in range(n)]
def list_to_str(l, sep=' '): return sep.join(map(str, l))
l2s = list_to_str
# shape: tuple of ints | list of ints
def make_list(shape, value=None):
    if len(shape) == 1:
        return [value] * shape[0]
    return [make_list(shape[1:], value) for _ in range(shape[0])]

if __name__ == '__main__':
    # sys.setrecursionlimit(1000000)
    main()


Fixed submission:
# import sys
from sys import stdout
# from copy import copy, deepcopy
# from functools import lru_cache
# from string import ascii_lowercase
# from math import inf
# inf = float('inf')


def main():
    ans = solve_case()
    stdout.write("{}\n".format(ans))

def solve_case():
    S = read_str()

    len_s = len(S)
    int_s = [ord(c) - ord('a') for c in S]

    # next_char_pos[from_idx][letter_idx] := the position of the next letter `letter_idx` from `from_idx`
    next_char_pos = make_list((len_s + 1, 26), len_s)
    for from_idx in reversed(range(len_s)):
        for letter_idx in range(26):
            if int_s[from_idx] == letter_idx:
                pos = from_idx
            else:
                pos = next_char_pos[from_idx+1][letter_idx]
            next_char_pos[from_idx][letter_idx] = pos

    # non_subseq_len[from_idx] := the length of the shortest "non subsequence" in S[from_idx:]
    non_subseq_len = make_list([len_s + 2], len_s + 1)
    non_subseq_len[len_s] = 1
    non_subseq_len[len_s + 1] = 0
    ans_next_pos = make_list([len_s + 1], len_s)
    ans_letter = make_list([len_s + 1], -1)
    for from_idx in reversed(range(len_s)):
        for letter_idx in range(26):
            new_len = non_subseq_len[next_char_pos[from_idx][letter_idx] + 1] + 1
            if non_subseq_len[from_idx] > new_len:
                non_subseq_len[from_idx] = new_len
                ans_letter[from_idx] = letter_idx
                ans_next_pos[from_idx] = next_char_pos[from_idx][letter_idx] + 1

    ans = ''
    idx = 0
    while idx < len(S):
        ans += chr(ord('a') + ans_letter[idx])
        idx = ans_next_pos[idx]
    return ans

#################################

def read_str(): return input()
def read_int(): return int(input())
def read_str_list(): return input().split(' ')
def read_int_list(): return list(map(int, input().split(' ')))
def read_lines(n, read_func): return [read_func() for _ in range(n)]
def list_to_str(l, sep=' '): return sep.join(map(str, l))
l2s = list_to_str
# shape: tuple of ints | list of ints
def make_list(shape, value=None):
    if len(shape) == 1:
        return [value] * shape[0]
    return [make_list(shape[1:], value) for _ in range(shape[0])]

if __name__ == '__main__':
    # sys.setrecursionlimit(1000000)
    main()

Buggy python submission (verdict: Time Limit Exceeded):
import sys
from copy import copy, deepcopy
from functools import lru_cache
from string import ascii_lowercase
# from math import inf
inf = float('inf')


def noop(*object): pass
# print = noop # use print() to debug and uncomment this line to mute it

def main():
    ans = solve_case()
    sys.stdout.write("{}\n".format(ans))

def solve_case():
    S = read_str()

    @lru_cache(maxsize=None)
    # the position of the first `char` from `from_idx`
    def next_char_pos(from_idx, char):
        if from_idx == len(S):
            return len(S)
        if S[from_idx] == char:
            return from_idx
        return next_char_pos(from_idx+1, char)

    recon = dict()

    @lru_cache(maxsize=None)
    # the length of the shortest "non subsequence" in S[from_idx:]
    def non_subseq_len(from_idx):
        if from_idx == len(S):
            return 1
        if from_idx > len(S):
            return 0
        ans = inf
        for letter in ascii_lowercase:
            new_ans = non_subseq_len(next_char_pos(from_idx, letter) + 1) + 1
            if ans > new_ans:
                ans = new_ans
                recon[from_idx] = (letter, next_char_pos(from_idx, letter) + 1)
        return ans

    for from_idx in reversed(range(len(S))):
        non_subseq_len(from_idx)

    ans = ''
    idx = 0
    while idx < len(S):
        letter, next_idx = recon[idx]
        ans += letter
        idx = next_idx
    return ans

#################################

def read_str(): return input()
def read_int(): return int(input())
def read_str_list(): return input().split(' ')
def read_int_list(): return list(map(int, input().split(' ')))
def read_lines(n, read_func): return [read_func() for _ in range(n)]
def list_to_str(l, sep=' '): return sep.join(map(str, l))
l2s = list_to_str
# shape: tuple of ints | list of ints
def make_list(shape, value=None):
    if len(shape) == 1:
        return [value] * shape[0]
    return [make_list(shape[1:], value) for _ in range(shape[0])]

if __name__ == '__main__':
    # sys.setrecursionlimit(1000000)
    main()


Fixed submission:
# import sys
from sys import stdout
# from copy import copy, deepcopy
# from functools import lru_cache
# from string import ascii_lowercase
# from math import inf
# inf = float('inf')


def main():
    ans = solve_case()
    stdout.write("{}\n".format(ans))

def solve_case():
    S = read_str()

    len_s = len(S)
    int_s = [ord(c) - ord('a') for c in S]

    # next_char_pos[from_idx][letter_idx] := the position of the next letter `letter_idx` from `from_idx`
    next_char_pos = make_list((len_s + 1, 26), len_s)
    for from_idx in reversed(range(len_s)):
        for letter_idx in range(26):
            if int_s[from_idx] == letter_idx:
                pos = from_idx
            else:
                pos = next_char_pos[from_idx+1][letter_idx]
            next_char_pos[from_idx][letter_idx] = pos

    # non_subseq_len[from_idx] := the length of the shortest "non subsequence" in S[from_idx:]
    non_subseq_len = make_list([len_s + 2], len_s + 1)
    non_subseq_len[len_s] = 1
    non_subseq_len[len_s + 1] = 0
    ans_next_pos = make_list([len_s + 1], len_s)
    ans_letter = make_list([len_s + 1], -1)
    for from_idx in reversed(range(len_s)):
        for letter_idx in range(26):
            new_len = non_subseq_len[next_char_pos[from_idx][letter_idx] + 1] + 1
            if non_subseq_len[from_idx] > new_len:
                non_subseq_len[from_idx] = new_len
                ans_letter[from_idx] = letter_idx
                ans_next_pos[from_idx] = next_char_pos[from_idx][letter_idx] + 1

    ans = ''
    idx = 0
    while idx < len(S):
        ans += chr(ord('a') + ans_letter[idx])
        idx = ans_next_pos[idx]
    return ans

#################################

def read_str(): return input()
def read_int(): return int(input())
def read_str_list(): return input().split(' ')
def read_int_list(): return list(map(int, input().split(' ')))
def read_lines(n, read_func): return [read_func() for _ in range(n)]
def list_to_str(l, sep=' '): return sep.join(map(str, l))
l2s = list_to_str
# shape: tuple of ints | list of ints
def make_list(shape, value=None):
    if len(shape) == 1:
        return [value] * shape[0]
    return [make_list(shape[1:], value) for _ in range(shape[0])]

if __name__ == '__main__':
    # sys.setrecursionlimit(1000000)
    main()

Buggy python submission (verdict: Runtime Error):
from sys import stdout
# from copy import copy, deepcopy
from functools import lru_cache
from string import ascii_lowercase
from math import inf


def noop(*object): pass
# print = noop # use print() to debug and uncomment this line to mute it

def main():
    ans = solve_case()
    stdout.write("{}\n".format(ans))

def solve_case():
    S = read_str()

    @lru_cache(maxsize=None)
    # the position of the first `char` from `from_idx`
    def next_char_pos(from_idx, char):
        if from_idx == len(S):
            return len(S)
        if S[from_idx] == char:
            return from_idx
        return next_char_pos(from_idx+1, char)

    recon = dict()

    @lru_cache(maxsize=None)
    # the length of the shortest "non subsequence" in S[from_idx:]
    def non_subseq_len(from_idx):
        if from_idx == len(S):
            return 1
        if from_idx > len(S):
            return 0
        ans = inf
        for letter in ascii_lowercase:
            new_ans = non_subseq_len(next_char_pos(from_idx, letter) + 1) + 1
            if ans > new_ans:
                ans = new_ans
                recon[from_idx] = (letter, next_char_pos(from_idx, letter) + 1)
        return ans

    for from_idx in reversed(range(len(S))):
        non_subseq_len(from_idx)

    ans = ''
    idx = 0
    while idx < len(S):
        letter, next_idx = recon[idx]
        ans += letter
        idx = next_idx
    return ans

#################################

def read_str(): return input()
def read_int(): return int(input())
def read_str_list(): return input().split(' ')
def read_int_list(): return list(map(int, input().split(' ')))
def read_lines(n, read_func): return [read_func() for _ in range(n)]
def list_to_str(l, sep=' '): return sep.join(map(str, l))
l2s = list_to_str
# shape: tuple of ints | list of ints
def make_list(shape, value=None):
    if len(shape) == 1:
        return [value] * shape[0]
    return [make_list(shape[1:], value) for _ in range(shape[0])]

if __name__ == '__main__':
    # sys.setrecursionlimit(1000000)
    main()


Fixed submission:
# import sys
from sys import stdout
# from copy import copy, deepcopy
# from functools import lru_cache
# from string import ascii_lowercase
# from math import inf
# inf = float('inf')


def main():
    ans = solve_case()
    stdout.write("{}\n".format(ans))

def solve_case():
    S = read_str()

    len_s = len(S)
    int_s = [ord(c) - ord('a') for c in S]

    # next_char_pos[from_idx][letter_idx] := the position of the next letter `letter_idx` from `from_idx`
    next_char_pos = make_list((len_s + 1, 26), len_s)
    for from_idx in reversed(range(len_s)):
        for letter_idx in range(26):
            if int_s[from_idx] == letter_idx:
                pos = from_idx
            else:
                pos = next_char_pos[from_idx+1][letter_idx]
            next_char_pos[from_idx][letter_idx] = pos

    # non_subseq_len[from_idx] := the length of the shortest "non subsequence" in S[from_idx:]
    non_subseq_len = make_list([len_s + 2], len_s + 1)
    non_subseq_len[len_s] = 1
    non_subseq_len[len_s + 1] = 0
    ans_next_pos = make_list([len_s + 1], len_s)
    ans_letter = make_list([len_s + 1], -1)
    for from_idx in reversed(range(len_s)):
        for letter_idx in range(26):
            new_len = non_subseq_len[next_char_pos[from_idx][letter_idx] + 1] + 1
            if non_subseq_len[from_idx] > new_len:
                non_subseq_len[from_idx] = new_len
                ans_letter[from_idx] = letter_idx
                ans_next_pos[from_idx] = next_char_pos[from_idx][letter_idx] + 1

    ans = ''
    idx = 0
    while idx < len(S):
        ans += chr(ord('a') + ans_letter[idx])
        idx = ans_next_pos[idx]
    return ans

#################################

def read_str(): return input()
def read_int(): return int(input())
def read_str_list(): return input().split(' ')
def read_int_list(): return list(map(int, input().split(' ')))
def read_lines(n, read_func): return [read_func() for _ in range(n)]
def list_to_str(l, sep=' '): return sep.join(map(str, l))
l2s = list_to_str
# shape: tuple of ints | list of ints
def make_list(shape, value=None):
    if len(shape) == 1:
        return [value] * shape[0]
    return [make_list(shape[1:], value) for _ in range(shape[0])]

if __name__ == '__main__':
    # sys.setrecursionlimit(1000000)
    main()

Buggy python submission (verdict: Runtime Error):
import sys
from copy import copy, deepcopy
from functools import lru_cache
from string import ascii_lowercase
from math import inf


def noop(*object): pass
# print = noop # use print() to debug and uncomment this line to mute it

def main():
    ans = solve_case()
    sys.stdout.write("{}\n".format(ans))

def solve_case():
    S = read_str()

    @lru_cache(maxsize=None)
    # the position of the first `char` from `from_idx`
    def next_char_pos(from_idx, char):
        if from_idx == len(S):
            return len(S)
        if S[from_idx] == char:
            return from_idx
        return next_char_pos(from_idx+1, char)

    recon = dict()

    @lru_cache(maxsize=None)
    # the length of the shortest "non subsequence" in S[from_idx:]
    def non_subseq_len(from_idx):
        if from_idx == len(S):
            return 1
        if from_idx > len(S):
            return 0
        ans = inf
        for letter in ascii_lowercase:
            new_ans = non_subseq_len(next_char_pos(from_idx, letter) + 1) + 1
            if ans > new_ans:
                ans = new_ans
                recon[from_idx] = (letter, next_char_pos(from_idx, letter) + 1)
        return ans

    for from_idx in reversed(range(len(S))):
        non_subseq_len(from_idx)

    ans = ''
    idx = 0
    while idx < len(S):
        letter, next_idx = recon[idx]
        ans += letter
        idx = next_idx
    return ans

#################################

def read_str(): return input()
def read_int(): return int(input())
def read_str_list(): return input().split(' ')
def read_int_list(): return list(map(int, input().split(' ')))
def read_lines(n, read_func): return [read_func() for _ in range(n)]
def list_to_str(l, sep=' '): return sep.join(map(str, l))
l2s = list_to_str
# shape: tuple of ints | list of ints
def make_list(shape, value=None):
    if len(shape) == 1:
        return [value] * shape[0]
    return [make_list(shape[1:], value) for _ in range(shape[0])]

if __name__ == '__main__':
    # sys.setrecursionlimit(1000000)
    main()


Fixed submission:
# import sys
from sys import stdout
# from copy import copy, deepcopy
# from functools import lru_cache
# from string import ascii_lowercase
# from math import inf
# inf = float('inf')


def main():
    ans = solve_case()
    stdout.write("{}\n".format(ans))

def solve_case():
    S = read_str()

    len_s = len(S)
    int_s = [ord(c) - ord('a') for c in S]

    # next_char_pos[from_idx][letter_idx] := the position of the next letter `letter_idx` from `from_idx`
    next_char_pos = make_list((len_s + 1, 26), len_s)
    for from_idx in reversed(range(len_s)):
        for letter_idx in range(26):
            if int_s[from_idx] == letter_idx:
                pos = from_idx
            else:
                pos = next_char_pos[from_idx+1][letter_idx]
            next_char_pos[from_idx][letter_idx] = pos

    # non_subseq_len[from_idx] := the length of the shortest "non subsequence" in S[from_idx:]
    non_subseq_len = make_list([len_s + 2], len_s + 1)
    non_subseq_len[len_s] = 1
    non_subseq_len[len_s + 1] = 0
    ans_next_pos = make_list([len_s + 1], len_s)
    ans_letter = make_list([len_s + 1], -1)
    for from_idx in reversed(range(len_s)):
        for letter_idx in range(26):
            new_len = non_subseq_len[next_char_pos[from_idx][letter_idx] + 1] + 1
            if non_subseq_len[from_idx] > new_len:
                non_subseq_len[from_idx] = new_len
                ans_letter[from_idx] = letter_idx
                ans_next_pos[from_idx] = next_char_pos[from_idx][letter_idx] + 1

    ans = ''
    idx = 0
    while idx < len(S):
        ans += chr(ord('a') + ans_letter[idx])
        idx = ans_next_pos[idx]
    return ans

#################################

def read_str(): return input()
def read_int(): return int(input())
def read_str_list(): return input().split(' ')
def read_int_list(): return list(map(int, input().split(' ')))
def read_lines(n, read_func): return [read_func() for _ in range(n)]
def list_to_str(l, sep=' '): return sep.join(map(str, l))
l2s = list_to_str
# shape: tuple of ints | list of ints
def make_list(shape, value=None):
    if len(shape) == 1:
        return [value] * shape[0]
    return [make_list(shape[1:], value) for _ in range(shape[0])]

if __name__ == '__main__':
    # sys.setrecursionlimit(1000000)
    main()

Buggy python submission (verdict: Runtime Error):
import sys
from copy import copy, deepcopy
from functools import lru_cache
from string import ascii_lowercase
from math import inf


def noop(*object): pass
# print = noop # use print() to debug and uncomment this line to mute it

def main():
    ans = solve_case()
    sys.stdout.write("{}\n".format(ans))

def solve_case():
    S = read_str()

    @lru_cache(maxsize=None)
    # the position of the first `char` from `from_idx`
    def next_char_pos(from_idx, char):
        if from_idx == len(S):
            return len(S)
        if S[from_idx] == char:
            return from_idx
        return next_char_pos(from_idx+1, char)

    recon = dict()

    @lru_cache(maxsize=None)
    # the length of the shortest "non subsequence" in S[from_idx:]
    def non_subseq_len(from_idx):
        if from_idx == len(S):
            return 1
        if from_idx > len(S):
            return 0
        ans = inf
        for letter in ascii_lowercase:
            new_ans = non_subseq_len(next_char_pos(from_idx, letter) + 1) + 1
            if ans > new_ans:
                ans = new_ans
                recon[from_idx] = (letter, next_char_pos(from_idx, letter) + 1)
        return ans

    for from_idx in reversed(range(len(S))):
        non_subseq_len(from_idx)

    ans = ''
    idx = 0
    while idx < len(S):
        letter, next_idx = recon[idx]
        ans += letter
        idx = next_idx
    return ans

#################################

def read_str(): return input()
def read_int(): return int(input())
def read_str_list(): return input().split(' ')
def read_int_list(): return list(map(int, input().split(' ')))
def read_lines(n, read_func): return [read_func() for _ in range(n)]
def list_to_str(l, sep=' '): return sep.join(map(str, l))
l2s = list_to_str
# shape: tuple of ints | list of ints
def make_list(shape, value=None):
    if len(shape) == 1:
        return [value] * shape[0]
    return [make_list(shape[1:], value) for _ in range(shape[0])]

if __name__ == '__main__':
    sys.setrecursionlimit(1000000)
    main()


Fixed submission:
# import sys
from sys import stdout
# from copy import copy, deepcopy
# from functools import lru_cache
# from string import ascii_lowercase
# from math import inf
# inf = float('inf')


def main():
    ans = solve_case()
    stdout.write("{}\n".format(ans))

def solve_case():
    S = read_str()

    len_s = len(S)
    int_s = [ord(c) - ord('a') for c in S]

    # next_char_pos[from_idx][letter_idx] := the position of the next letter `letter_idx` from `from_idx`
    next_char_pos = make_list((len_s + 1, 26), len_s)
    for from_idx in reversed(range(len_s)):
        for letter_idx in range(26):
            if int_s[from_idx] == letter_idx:
                pos = from_idx
            else:
                pos = next_char_pos[from_idx+1][letter_idx]
            next_char_pos[from_idx][letter_idx] = pos

    # non_subseq_len[from_idx] := the length of the shortest "non subsequence" in S[from_idx:]
    non_subseq_len = make_list([len_s + 2], len_s + 1)
    non_subseq_len[len_s] = 1
    non_subseq_len[len_s + 1] = 0
    ans_next_pos = make_list([len_s + 1], len_s)
    ans_letter = make_list([len_s + 1], -1)
    for from_idx in reversed(range(len_s)):
        for letter_idx in range(26):
            new_len = non_subseq_len[next_char_pos[from_idx][letter_idx] + 1] + 1
            if non_subseq_len[from_idx] > new_len:
                non_subseq_len[from_idx] = new_len
                ans_letter[from_idx] = letter_idx
                ans_next_pos[from_idx] = next_char_pos[from_idx][letter_idx] + 1

    ans = ''
    idx = 0
    while idx < len(S):
        ans += chr(ord('a') + ans_letter[idx])
        idx = ans_next_pos[idx]
    return ans

#################################

def read_str(): return input()
def read_int(): return int(input())
def read_str_list(): return input().split(' ')
def read_int_list(): return list(map(int, input().split(' ')))
def read_lines(n, read_func): return [read_func() for _ in range(n)]
def list_to_str(l, sep=' '): return sep.join(map(str, l))
l2s = list_to_str
# shape: tuple of ints | list of ints
def make_list(shape, value=None):
    if len(shape) == 1:
        return [value] * shape[0]
    return [make_list(shape[1:], value) for _ in range(shape[0])]

if __name__ == '__main__':
    # sys.setrecursionlimit(1000000)
    main()