Multiply two strings

Given two numbers as strings s1 and s2. Calculate their Product.

Note: The numbers can be negative.

• Time : O(nm)
• Space : O(n + m)

class Solution{
public:
    /*You are required to complete below function */
    string multiplyStrings(string s1, string s2) {
        reverse(begin(s1), end(s1));
        reverse(begin(s2), end(s2));
        int sign = 1, n = s1.length(), m = s2.length();
        if(s1.back() == '-') {
            sign = -sign;
            s1.pop_back();
        }
        if(s2.back() == '-') {
            sign = -sign;
            s2.pop_back();
        }
        string res(n + m, '0');
        
        for(int i = 0; i < n; i++) {
            int carry = 0;
            for(int j = 0; j < m; j++) {
                int mul = carry + (s1[i] & 0b1111) * (s2[j] & 0b1111) + (res[i + j] & 0b1111);
                carry = mul / 10;
                mul = mul % 10;
                res[i + j] = (mul | 0b110000);
            }
            int j = m;
            while(carry) {
                int sum = carry + (res[i + j] & 0b1111);
                carry = sum / 10;
                sum = sum % 10;
                res[i + j] = (sum | 0b110000);
            }
        }
        
        while(!res.empty() and res.back() == '0') res.pop_back();
        if(!res.empty() and sign == -1) res.push_back('-');
        reverse(begin(res), end(res));
        return res.empty() ? "0" : res;
    }
};