leetcode-415：字符串相加

class Solution:
    def addStrings(self, num1: str, num2: str) -> str:
        i = len(num1)-1
        j = len(num2)-1
        add = 0
        ans = ""
        while i>=0 or j>=0 or add:
            x = int(num1[i]) if i>=0 else 0
            y = int(num2[j]) if j>=0 else 0
            add += x+y
            ans += str(add%10)
            add//=10
            i-=1
            j-=1
        return ans[::-1]