原题地址：https://oj.leetcode.com/problems/plus-one/

题意：

Given a non-negative number represented as an array of digits, plus one to the number.

The digits are stored such that the most significant digit is at the head of the list.

解题思路：这只一个flag标志位就可以了。

代码：

class Solution:
    # @param digits, a list of integer digits
    # @return a list of integer digits
    def plusOne(self, digits):
        flag = 1
        for i in range(len(digits)-1, -1, -1):
            if digits[i] + flag == 10:
                digits[i] = 0
                flag = 1
            else:
                digits[i] = digits[i] + flag
                flag = 0
        
        if flag == 1:
            digits.insert(0, 1)
        return digits