Maximum 69 number

We will iterate over all the digits of the number and check if the digit is 6. If it is, we will replace it with 9 and return the number.

class Solution:
    def maximum69Number (self, num: int) -> int:
        num = list(str(num))
        for i in range(len(num)):
            if num[i] == '6':
                num[i] = '9'
                break
        return int(''.join(num))

Time complexity: O(n) Space complexity: O(n)

Here is a one-liner solution:

class Solution:
    def maximum69Number (self, num: int) -> int:
        return int(str(num).replace('6', '9', 1))