10106 -Product.cpp

#include<iostream>
#include<string>
#include<sstream>
#include<cstdlib>
#include<vector>
#include<algorithm>

using namespace std;

int carry(int sum)
{
    return sum/10;
}

int digit(int sum)
{
    return sum%10;
}

string add (vector<string>BigInteger)
{
    int maxLength = 0;
    for(int i = 0; i < BigInteger.size(); i++)
    {
        if(BigInteger[i].length() >= maxLength) maxLength = BigInteger[i].length();
    }

    for(int i = 0; i < BigInteger.size(); i++)
    {
        int l = BigInteger[i].length();
        for(int j = 0; j < maxLength-l; j++)
        {
            BigInteger[i] = "0" + BigInteger[i];
        }

    }

    int hateAche = 0;
    string result;
    for(int i = maxLength-1; i >= 0; i--)
    {
        int sum = hateAche;
        for(int j = 0; j < BigInteger.size(); j++)
        {
            int d = BigInteger[j][i] - '0';
            sum += d;
        }

        hateAche = carry(sum);

        char ch = digit(sum) + '0';
        result = ch + result;
    }

    if(hateAche > 0)
    {
        stringstream ss;
        ss << hateAche;
        string hateAcheString;
        ss >> hateAcheString;
        result = hateAcheString + result;
    }

    return result;
}

string multiply(string str, char ch)
{
    int hateAche = 0;
    int mul = ch - '0';
    string result;
    for(int i = str.length()-1; i >= 0; i--)
    {
        int dig = str[i] - '0';
        dig = dig*mul + hateAche;
        ch = digit(dig) + '0';
        hateAche = carry(dig);
        result = ch + result;
    }

    if(hateAche > 0)
    {
        stringstream ss;
        ss << hateAche;
        string hateAcheString;
        ss >> hateAcheString;
        result = hateAcheString + result;
    }

    return result;
}

string removeLeadingZero(string str)
{
    string a;
    for(int i = 0; i < str.length(); i++)
    {
        if(str[i] == '0') str[i] = ' ';
        else break;
    }

    for(int i = 0; i < str.length(); i++)
    {
        if(str[i] == ' ');
        else a += str[i];
    }

    if(a.length() == 0) return "0";
    else return a;
}


int main (void)
{
    string a, b;
    while(cin >> a >> b)
    {
        int n = 0;
        vector<string> BigInteger;
        for(int i = b.length()-1; i >= 0; i--)
        {
            string mul = multiply(a, b[i]);
            for(int j = 0; j < n; j++)
            {
                char ch = '0';
                mul += ch;
            }
            n++;
            BigInteger.push_back(mul);

        }

        string res = add(BigInteger);
        res = removeLeadingZero(res);
        cout << res << endl;
    }

    return 0;
}