Buggy python submission (verdict: Time Limit Exceeded):
N = int(input())

#文字数分aで埋める
#末尾をbにする
#一桁上の文字+1文字までは許容するルールで増加させていく

import itertools

stop_combination = [chr(i) for i in range(97,97+N)]
combination_list = list(itertools.product(stop_combination, repeat=N))

printed_expression = set()

for combination in combination_list:
    #print(combination,":",end="")

    if list(combination) == stop_combination:
        print(''.join(map(str, combination)))
        break

    character_dict = dict()
    character_number = 0
    alternative_expression = ""

    for character in combination:
        if character not in character_dict:
            character_dict[character] = character_number
            alternative_expression += str(character_number)
            character_number += 1
        else:
            alternative_expression += str(character_dict[character])

    #print(alternative_expression)

    if alternative_expression not in printed_expression:
        print(''.join(map(str, combination)))
        printed_expression.add(alternative_expression)

Fixed submission:
N = int(input())

#文字数分aで埋める
#末尾をbにする
#一桁上の文字+1文字までは許容するルールで増加させていく

ans = [0 for _ in range(N)]
stop = [i for i in range(N)]

def ans_print(ans_list):
    ans = list()
    for a in ans_list:
        ans.append(chr(a+97)) #97でaが出る

    print(''.join(map(str, ans)))
    return 0

def ans_check(ans):
    max_in_ans = 0

    for i in range(1,N):
        if max_in_ans + 1 < ans[i]:
            ans[i-1] += 1
            ans[i] = 0
            return ans_check(ans)
        else:
            max_in_ans = max(max_in_ans,ans[i])
    else:
        return ans

if N == 1:
    print("a")
else:
    while 1:
        ans_check(ans)
        ans_print(ans)
        ans[-1] += 1
        if ans == stop:
            ans_print(ans)
            break

Buggy python submission (verdict: Time Limit Exceeded):
N = int(input())

#文字数分aで埋める
#末尾をbにする
#一桁上の文字+1文字までは許容するルールで増加させていく

import itertools

stop_combination = [chr(i) for i in range(97,97+N)]
combination_list = list(itertools.product(stop_combination, repeat=N))

printed_expression = set()

for combination in combination_list:
    #print(combination,":",end="")

    if list(combination) == stop_combination:
        print(''.join(map(str, combination)))
        break

    character_dict = dict()
    character_number = 0
    alternative_expression = ""

    for character in combination:
        if character not in character_dict:
            character_dict[character] = character_number
            alternative_expression += str(character_number)
            character_number += 1
        else:
            alternative_expression += str(character_dict[character])

    #print(alternative_expression)

    if alternative_expression not in printed_expression:
        print(''.join(map(str, combination)))
        printed_expression.add(alternative_expression)

Fixed submission:
N = int(input())

#文字数分aで埋める
#末尾をbにする
#一桁上の文字+1文字までは許容するルールで増加させていく

ans = [0 for _ in range(N)]
stop = [i for i in range(N)]

def ans_print(ans_list):
    ans = list()
    for a in ans_list:
        ans.append(chr(a+97)) #97でaが出る

    print(''.join(map(str, ans)))
    return 0

def ans_check(ans):
    max_in_ans = 0

    for i in range(1,N):
        if max_in_ans + 1 < ans[i]:
            ans[i-1] += 1
            ans[i] = 0
            return ans_check(ans)
        else:
            max_in_ans = max(max_in_ans,ans[i])
    else:
        return ans

if N == 1:
    print("a")
else:
    while 1:
        ans_check(ans)
        ans_print(ans)
        ans[-1] += 1
        if ans == stop:
            ans_print(ans)
            break

Buggy python submission (verdict: Runtime Error):
N = int(input())

#文字数分aで埋める
#末尾をbにする
#一桁上の文字+1文字までは許容するルールで増加させていく

ans = [0 for _ in range(N)]
stop = [i for i in range(N)]

def ans_print(ans_list):
    ans = list()
    for a in ans_list:
        ans.append(chr(a+97)) #97でaが出る

    print(''.join(map(str, ans)))
    return 0

while 1:
    ans_print(ans)
    
    ans[-1] += 1

    for i in range(N-1,0,-1):
        if ans[i] > ans[i-1]+1:
            ans[i-1] += 1
            ans[i] = 0
            
    if ans == stop:
        ans_print(ans)
        break


Fixed submission:
N = int(input())

#文字数分aで埋める
#末尾をbにする
#一桁上の文字+1文字までは許容するルールで増加させていく

ans = [0 for _ in range(N)]
stop = [i for i in range(N)]

def ans_print(ans_list):
    ans = list()
    for a in ans_list:
        ans.append(chr(a+97)) #97でaが出る

    print(''.join(map(str, ans)))
    return 0

def ans_check(ans):
    max_in_ans = 0

    for i in range(1,N):
        if max_in_ans + 1 < ans[i]:
            ans[i-1] += 1
            ans[i] = 0
            return ans_check(ans)
        else:
            max_in_ans = max(max_in_ans,ans[i])
    else:
        return ans

if N == 1:
    print("a")
else:
    while 1:
        ans_check(ans)
        ans_print(ans)
        ans[-1] += 1
        if ans == stop:
            ans_print(ans)
            break