leetcode 415. Add Strings

class Solution {public:    string addStrings(string num1, string num2) {        string &a =num1;        string &b = num2;        if(a.length() < b.length())            swap(a,b);        int x1 = a.length()-1,x2 = b.length()-1;        for(;x2 >= 0  ;x1--,x2--){            a[x1] = b[x2] + a[x1] - '0';            if(x1 > 0 && a[x1] > '9'){                a[x1-1]++;                a[x1] -= 10;            }        }        while(x1 > 0){            if(a[x1] > '9'){                a[x1] -= 10;                a[x1-1]++;            }            else break;            x1--;        }        if(a[0] > '9'){            a[0] = a[0] - 10;            a = '1'+a;        }        return a;    }};