Leetcode 43: Multiply Strings

leetcode_43.cpp

#include <iostream>
#include <string>
#include <vector>

using namespace std;

string multiply(string num1, string num2) {
    if (num1.size() < num2.size())
        swap(num1, num2);
    vector<string> a;
    for (int i=num2.size()-1; i>=0; i--) {
        string sum(num1.size()+num2.size(), '0');
        int carry=0, j=0;
        for (j=num1.size()-1; j>=0; j--) {
            int digit = (num2[i]-'0')*(num1[j]-'0');
            digit += carry;
            int pos = (num2.size()-i-1)+(num1.size()-j-1);
            sum[sum.size()-pos-1] = '0'+(digit%10);
            carry = (digit/10);
        }
        if (carry>0) {
            int pos = num1.size()+(num2.size()-i); //pos from right
            sum[sum.size()-pos] = carry + '0';
        }
        a.push_back(sum);
    }
    //now add all the string numbers in array a
    string result(num1.size() + num2.size(), '0');
    int carry = 0;
    for (int i=result.size()-1; i>=0; i--) {
        int digit = carry;
        for (int j=0; j<a.size(); j++)
            digit += a[j][i] - '0';
        result[i] = (digit % 10) + '0';
        carry = digit / 10;
    }
    //strip out leading zeros
    int start = 0;
    for ( ; start < result.size()-1 && result[start]=='0'; start++) {}
    return result.substr(start);
}

int main() {
    string num1 = "999", num2 = "999";
    cout << multiply(num1, num2) << endl;
    return 0;
}