leetcode 415 字符串相加

模拟加法处理过程，由于两数相加是从右往左进位，这里需要将两个字符串先反转之后，再进行逐位相加处理

class Solution {
public:
    string addStrings(string num1, string num2) {
        string ans;
        reverse(num1.begin(), num1.end());
        reverse(num2.begin(), num2.end());

        int n = max(num1.size(), num2.size()), carry = 0;
        for (int i = 0; i < n; ++i) {
            carry += i < num1.size() ? (num1.at(i) - '0') : 0;
            carry += i < num2.size() ? (num2.at(i) - '0') : 0;
            ans.push_back('0' + (carry % 10));
            carry /= 10;
        }
        if (carry) {
            ans.push_back('1');
        }
        reverse(ans.begin(), ans.end());
        return ans;
    }
};