Add Strings

Given two non-negative numbers num1 and num2 represented as string, return the sum of num1 and num2.
编程答案：

class Solution {public:    string addStrings(string num1, string num2) {        if(num1.size()-1 < num2.size()-1) //第一个数的长度大于第二个数。        {           return addStrings(num2,num1);        }        int i,j;        int addBits =0;        for(i=num1.size()-1,j= num2.size()-1;i>=0||j>=0;addBits/=10)        {        if(j>=0)        {            addBits += num1[i--]-'0'+num2[j--]-'0';            num1[i+1] = addBits%10 + '0';        }        else        {         addBits += num1[i--]-'0';         num1[i+1] = addBits%10 + '0';        }            }        return addBits?"1"+num1:num1;    }};