leetcode 日经贴，Cpp code -Multiply Strings

Multiply Strings

class Solution {
public:
    string multiplyPositive(string num1, string num2) {
        reverse(num1.begin(), num1.end());
        reverse(num2.begin(), num2.end());
        int n = num1.length(), m = num2.length();
        vector<int> product(n + m);
        int carry = 0;
        for (int i = 0; i < n; ++i) {
            for (int j = 0; j < m; ++j) {
                carry += (num1[i] - '0') * (num2[j] - '0') + product[i + j];
                product[i + j] = carry % 10;
                carry /= 10;
            }
            product[i + m] = carry;
            carry = 0;
        }
        int k = n + m - 1;
        while (k >= 0 && !product[k]) {
            --k;
        }
        if (k < 0) {
            return "0";
        } else {
            string s;
            for (int i = k; i >= 0; --i) {
                s += char('0' + product[i]);
            }
            return s;
        }
    }
    
    string multiply(string num1, string num2) {
        string s1, s2;
        if (num1.length() == 0 || num2.length() == 0) {
            return "0";
        }
        bool neg = false;
        if (num1[0] == '-') {
            s1 = num1.substr(1, num1.length());
            neg = !neg;
        } else {
            s1 = num1;
        }
        if (num2[0] == '-') {
            s2 = num2.substr(1, num2.length());
            neg = !neg;
        } else {
            s2 = num2;
        }
        if (s1.length() == 0 || s2.length() == 0) {
            return "0";
        }
        string s = multiplyPositive(s1, s2);
        if (neg && s != "0") {
            s = '-' + s;
        }
        return s;
    }
};