Monotone Increasing Digits LeetCode solution in Python

1
# Runtime: 43 ms (Top 69.04%) | Memory: 13.9 MB (Top 20.57%)
2
class Solution:
3
    def monotoneIncreasingDigits(self, n: int) -> int:
4
        num = list(str(n))
5
        for i in range(len(num) - 1):
6
            # Step1: When don't meet the condition, num[i]-=1 and repalce all num left into '9' and directly return
7
            # However, there is the case that num[i-1]==num[i], which will make num[i]-1<num[i-1]
8
            # So, traverse back to find the num that its num[i-1] != num[i](to make sure num[i-1]<=num[i]-1), then do step1 and return
9
            if num[i] > num[i + 1]:
10
                while i >= 1 and num[i - 1] == num[i]:
11
                    i -= 1
12
                num[i] = chr(ord(num[i]) - 1)
13
                return int("".join(num[: i + 1] + ["9"] * (len(num) - i - 1)))
14
        return n