Maximum 69 Number — Python

# Time:  O(logn)
# Space: O(1)

class Solution(object):
    def maximum69Number (self, num):
        """
        :type num: int
        :rtype: int
        """
        curr, base, change = num, 3, 0
        while curr:
            if curr%10 == 6:
                change = base
            base *= 10
            curr //= 10
        return num+change


# Time:  O(logn)
# Space: O(logn)
class Solution2(object):
    def maximum69Number (self, num):
        """
        :type num: int
        :rtype: int
        """
        return int(str(num).replace('6', '9', 1))