Multiply Strings.txt

class Solution {
public:
    string multiply(string num1, string num2) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int size1 = num1.size(), size2 = num2.size(), size3 = size1 + size2;
        if (!size1 || !size2)
            return "";
        vector<int> n1, n2;
        for (int i = size1 - 1; i >= 0; i--)
            n1.push_back(num1[i] - '0');
        for (int i = size2 - 1; i >= 0; i--)
            n2.push_back(num2[i] - '0');
        vector<int> r1(size3+1, 0);
        multiply(r1, n1, n2, 0);
        for (int i = 1; i < num2.size(); i++) {
            vector<int> r2(size3+1, 0);
            multiply(r2, n1, n2, i);
            add(r1, r2);
        }
        while (r1.back() == 0)
            r1.pop_back();
        string ret;
        for (int i = r1.size() - 1; i >= 0; i--)
            ret += (char)('0' + r1[i]);
        return ret.empty()?"0":ret;
    }
    void multiply(vector<int>& r, vector<int>& n1, vector<int>& n2, int p) {
        int m = n2[p], c = 0;
        for (int i = 0; i < n1.size(); i++) {
            int sum = m*n1[i] + c;
            r[p++] = sum%10;
            c = sum/10;
        }
        if (c > 0)
            r[p] = c;
    }
    void add(vector<int>& r1, vector<int> r2) {
        int p = 0, c = 0;
        while (p < r1.size()) {
            int sum = r1[p] + r2[p] + c;
            r1[p] = sum%10;
            c = sum/10;
            p++;
        }
    }
};