multiply strings

#include <iostream>
#include <cstring>
#include <cstdlib>
#include <string>
#include <algorithm>
using namespace std;

string (string num1, string num2) {
    if((num1.size() == 1 || num2.size() == 1) && (num1[0] == '0' || num2[0] == '0')){
        return "0";
    }
    string result;
    int len1 = num1.size(), len2 = num2.size();
    int res[2000];
    int i, j;
    memset(res, 0, sizeof(res));

    for(i = 0; i < len1; i++){
        for(j = 0; j < len2; j++){
            res[i + j] += (num1[i] - '0') * (num2[j] - '0');
        }
    }
    int k = i + j - 2;  
    int goBit = 0;      //进位
    for(int i = k; i >= 0; i--){
        res[i + 1] = (res[i] + goBit) % 10;
        goBit = (res[i] + goBit) / 10;
    }
    if(goBit != 0){     //如果最后一次进位不为0
        res[0] = goBit;
        for(int i = 0; i <= k + 1; i++)
            result += (res[i] + '0');
    }else{
        for(int i = 1; i <= k + 1; i++)
            result += (res[i] + '0');
    }
    return result;
}




int main()
{

    cout << multiply("123", "12");
    return 0;
}