leetcode66 Plus One

"""
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.
"""
"""
解法一：从后向前迭代。当迭代到最高位时如果要进位
需要再次判断，运用了insert(a, b)函数。表示在位置a前插入b元素
"""
class Solution1:
    def plusOne(self, digits):
        carry = 0
        digits[len(digits)-1] += 1
        for i in range(len(digits)-1, -1, -1):
            if digits[i]+carry == 10:
                digits[i] = 0
                carry = 1
                if i == 0:
                    digits.insert(i, 1)
            else:
                digits[i] += carry
                carry = 0
        return digits
"""
解法二：递归
"""
class Solution2:
    def plusOne(self, digits):
        if len(digits) == 1 and digits[0] == 9:
            return [1, 0]
        if digits[-1] != 9:
            digits[-1] += 1
            return digits
        else:
            digits[-1] = 0
            digits[:-1] = self.plusOne(digits[:-1])
        return digits