leetcode1323

class Solution:
    def maximum69Number (self, num: int) -> int:
        result = ''
        str_num = str(num)
        i = 0
        while i < len(str_num):
            if str_num[i] == '6':
                result += '9'
                result += str_num[i+1:]
                return result
            else:
                result += str_num[i]
            i += 1
        return result

遍历数字，将第一个6变成9，然后返回。