https://editor.csdn.net/md?not_checkout=1&spm=1015.2103.3001.8066&articleId=138048516

class Solution {
public:
    string solve(string s, string t) {
        // write code here
        string ret;
        int tmp = 0; // 进位 + 本次累加和
        int i = s.size()-1, j = t.size()-1;
        while (i >= 0 || j >= 0 || tmp) {
            if (i >= 0) tmp += s[i--] - '0';
            if (j >= 0) tmp += t[j--] - '0';
            ret += '0' + tmp % 10;
            tmp /= 10;
        }
        reverse(ret.begin(), ret.end());
        return ret;
    }
};