大数加法

#include <iostream>

using namespace std;

string add(string str1, string str2)//大数加法
{
    string str;
    int len1 = str1.length();
    int len2 = str2.length();
    if(len1 < len2)
    {
        for(int i = 1; i <= len2 - len1; i++)
        {
            str1 = '0' + str1;
        }
    }
    else {
        for(int i = 1; i <= len1 - len2; i++)
        {
            str2 = "0" + str2;
        }
    }
    len1 = str1.length();
    int cmp = 0;
    int ans;
    for(int i = len1 - 1; i >= 0; i--)
    {
        ans = str1[i] - '0' + str2[i] - '0' + cmp;
        cmp = ans/10;
        ans %= 10;
        str = char(ans + '0') + str;
    }
    return str;
}

int main()
{
    string a, b;
    while(cin >> a >> b)
    {
        cout << add(a,b) << endl;
    }
    return 0;
}

 两个数均大于等于0