Add Strings

倒序归并就行了

class Solution(object):
    def addStrings(self, num1, num2):
        """
        :type num1: str
        :type num2: str
        :rtype: str
        """
        n, m = len(num1), len(num2)
        curN, curM = n - 1, m - 1
        step = 0
        result = ''
        while curN >= 0 and curM >= 0:
            tmp = ord(num1[curN]) + ord(num2[curM]) - 2 * ord('0') + step
            step, digit = divmod(tmp, 10)
            result = `digit` + result
            curN -= 1
            curM -= 1

        while curN >= 0:
            tmp = ord(num1[curN]) - ord('0') + step
            step, digit = divmod(tmp, 10)
            result = `digit` + result
            curN -= 1

        while curM >= 0:
            tmp = ord(num2[curM]) - ord('0') + step
            step, digit = divmod(tmp, 10)
            result = `digit` + result
            curM -= 1

        return result if not step else `step` + result