https://atcoder.jp/contests/abc171/tasks/abc171_c

abc171_c.py

# wrong answer...
import string

# 1千兆 + 1
N = int(input())
n_digits = len(str(1000000000000001))
letters = string.ascii_lowercase

x = N - 1
ans = []
while True:
    mod = x % 26
    ans = [ mod ] + ans
    x = x // 26 - 1
    if x <= 0:
        if ans[0] == 0:
            ans = [0] + ans
        break

# print(ans)
print("".join([ letters[v] for v in ans ]))

正解例

import string

# 1千兆 + 1
N = int(input())
n_digits = len(str(1000000000000001))
letters = string.ascii_lowercase

x = N - 1
ans = []

while True:
    mod = x % 26
    div = x // 26
    
    ans.append(mod)
    if div == 0:
        break
    x = div - 1

ans = ans[::-1]
# print(ans)
print("".join([ letters[v] for v in ans ]))

修正

import string

# 1千兆 + 1
N = int(input())
n_digits = len(str(1000000000000001))
letters = string.ascii_lowercase

x = N
ans = []
while True:
    x -= 1
    mod = x % 26
    x = x // 26
    
    ans.append(mod)
    if x == 0:
        break

ans = ans[::-1]
print("".join([ letters[v] for v in ans ]))