Leetcode 415. Add Strings

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

Note:

The length of both num1 and num2 is < 5100.
Both num1 and num2 contains only digits 0-9.
Both num1 and num2 does not contain any leading zero.
You must not use any built-in BigInteger library or convert the inputs to integer directly.

class Solution {
public:
    string addStrings(string num1, string num2) {
        //直接加就是了，右对齐
        //需要提取利用的信息:两个string的长度！！


        int n1=num1.size();
        int n2=num2.size();
        if(n1sum=num1;
        int j=n2-1;
        int carry=0;
        for(int i=n1-1;i>=0;i--){

            int cur=num1[i]-'0'+(j>=0?(num2[j]-'0'):0)+carry;
            sum[i]=cur%10 + '0';
            carry=cur>9?1:0;
            j--;
            if(j<0&&carry==0) return sum;
        }
        sum=carry==1?'1'+sum:sum;
        return sum;
    }
};