Given a non-empty array of digits representing a non-negative integer, plus one to the integer.

The digits are stored such that the most significant digit is at the head of the list, and each element in the array contain a single digit.

You may assume the integer does not contain any leading zero, except the number 0 itself.

Example 1:

Input: [1,2,3]
Output: [1,2,4]
Explanation: The array represents the integer 123.

Example 2:

Input: [4,3,2,1]
Output: [4,3,2,2]
Explanation: The array represents the integer 4321.

解法1，用加法思想：

class Solution(object):
    def plusOne(self, digits):
        """
        :type digits: List[int]
        :rtype: List[int]
        """
        # [0]=>[1]
        # [1]=>[2]
        # [9]=>[1, 0]
        # [1, 0]=>[1, 1]
        # [1, 9]=>[2, 0]
        # [9, 9]=>[1, 0, 0]
        # [1,2,3]=>[1,2,4]
        # xxxx9999
        """        
        i = len(digits)-1     
        while i >= 0 and digits[i]+1 > 9:
            digits[i] = digits[i]+1-10
            i -= 1
        if i < 0:
            digits.insert(0, 1)
        else:
            digits[i] += 1
        return digits
        

or:

class Solution(object):
    def plusOne(self, digits):
        """
        :type digits: List[int]
        :rtype: List[int]
        """
        # [0]=>[1]
        # [1]=>[2]
        # [9]=>[1, 0]
        # [1, 0]=>[1, 1]
        # [1, 9]=>[2, 0]
        # [9, 9]=>[1, 0, 0]
        # [1,2,3]=>[1,2,4]    
        rd = []
        carry = 0
        digits[-1] += 1
        for d in digits[::-1]:
            cs = carry + d
            rd.insert(0, cs % 10)
            carry = cs//10
        if carry:
            rd.insert(0, carry)
        return rd
        

解法2，本質上就是xxxx9999，找到第一個x：

class Solution(object):
    def plusOne(self, digits):
        """
        :type digits: List[int]
        :rtype: List[int]
        """
        # [0]=>[1]
        # [1]=>[2]
        # [9]=>[1, 0]
        # [1, 0]=>[1, 1]
        # [1, 9]=>[2, 0]
        # [9, 9]=>[1, 0, 0]
        # [1,2,3]=>[1,2,4]
        # xxxx9999
        i = len(digits)-1
        while i>=0 and digits[i] == 9:
            digits[i] = 0
            i -= 1
        if i >= 0:
            digits[i] += 1
        else:
            digits.insert(0, 1)
        return digits
        

leetcode 66. Plus One