Multiply Strings Solutions in C++

// Source : https://oj.leetcode.com/problems/multiply-strings/
// Author : Hao Chen
// Date   : 2014-07-18



class Solution {
    public:

        string strPlus(string& num1, string& num2) {
            if (num1.size()==0) return num2;
            if (num2.size()==0) return num1;

            if ( num1.size() < num2.size() ) {
                swap(num1, num2);
            }

            string s;
            int carry=0;
            int x;
            for (int i=num1.size()-1, j=num2.size()-1; i>=0; i--, j--) {
                x = num1[i]-'0' + carry;
                if(j>=0){
                    x += num2[j]-'0';
                }
                s.insert(s.begin(), x%10+'0');
                carry = x/10;
            }
            if (carry>0) {
                s.insert(s.begin(), carry+'0');
            }

            return s;
        }


        string multiply(string num1, string num2) {

            if (num1.size()<=0 || num2.size()<=0) return "";

            string result;
            for ( int i=num1.size()-1; i>=0; i--) {
                int carry = 0;
                string val;
                for( int j=num2.size()-1; j>=0; j--) {
                    int v = (num2[j]-'0') * (num1[i]-'0') + carry;
                    val.insert(val.begin(), v%10+'0');
                    carry = v/10;
                }
                if (carry) val.insert(val.begin(), carry+'0');
                for (unsigned int j=i; j<num1.size()-1; j++) {
                    val.push_back('0');
                }
                result = strPlus(result, val);
            }

            //check if it is zero
            if (result[0]=='0') return "0";
            return result;

        }
};



int main(int argc, char**argv)
{
    string s1="20";
    string s2="25";
    if (argc>2){
        s1 = argv[1];
        s2 = argv[2];
    }
    cout << s1 << " * " << s2 << " = " << multiply(s1, s2) << endl;
    return 0;
}

#include <string>
using std::string;

class Solution {
public:
    string multiply(string num1, string num2) {
        string ret(num1.size() + num2.size(), '0');
        for (int i = num1.size()-1; i >= 0; --i) {
            int carry = 0;
            for (int j = num2.size()-1; j >= 0; --j) {
                int sum = (ret[i+j+1]-'0') + (num1[i]-'0') * (num2[j]-'0') + carry;
                ret[i+j+1] = sum%10 + '0';
                carry = sum/10;
            }
            ret[i] += carry;
        }
        size_t pos = ret.find_first_not_of('0');
        if (pos != string::npos) return ret.substr(pos);
        else return "0";
    }
};