Multiply Large Numbers represented as Strings

// Function to multiply two numbers,
// represented as strings
using System;
using System.Text;

class GfG {

    // Function to multiply two numbers,
    // represented as strings
    static string multiplyStrings(string s1, string s2) {
        int n1 = s1.Length, n2 = s2.Length;
        if(n1 == 0 || n2 == 0)
            return "0";
        
        // check if string are negative
        int nn = 1, mm = 1;
        if(s1[0] == '-')
            nn = -1;
        if(s2[0] == '-')
            mm = -1;
        
        int isNeg = nn * mm;
        
        // will keep the result number in 
        // vector in reverse order
        int[] result = new int[n1 + n2];
        for (int i = 0; i < result.Length; i++)
            result[i] = 0;
        
        // index by s1
        int i1 = 0;
        
        // index by s2
        int i2 = 0;
        
        // go from right to left by s1
        for (int i = n1 - 1; i >= 0; i--) {
            if(s1[i] == '-')
                continue;
            int carry = 0;
            int n1Digit = s1[i] - '0';
            i2 = 0;
            
            // go from right to left by s2
            for (int j = n2 - 1; j >= 0; j--) {
                if(s2[j] == '-')
                    continue;
                int n2Digit = s2[j] - '0';
                
                // multiply and add this result 
                // to the existing result
                int sum = n1Digit * n2Digit + result[i1 + i2] + carry;
                
                // carry for next iteration
                carry = sum / 10;
                
                // store result
                result[i1 + i2] = sum % 10;
                
                i2++;
            }
            
            // store carry in next cell
            if(carry > 0)
                result[i1 + i2] += carry;
            
            i1++;
        }
        
        // ignore '0's from the right
        int k = result.Length - 1;
        while(k >= 0 && result[k] == 0)
            k--;
        
        // if all were '0's - means either 
        // both or one of s1 or s2 were '0'
        if(k == -1)
            return "0";
        
        // generate the result string
        StringBuilder s = new StringBuilder();
        while(k >= 0)
            s.Append(result[k--]);
        
        // if negative
        if(isNeg == -1)
            s.Insert(0, "-");
        
        return s.ToString();
    }
    
    // Function to add two numbers,
    // represented as strings
    static string addStrings(string num1, string num2) {
        StringBuilder result = new StringBuilder();
        int carry = 0;
        int p1 = num1.Length - 1, p2 = num2.Length - 1;
        while (p1 >= 0 || p2 >= 0) {
            int x1 = p1 >= 0 ? num1[p1] - '0' : 0;
            int x2 = p2 >= 0 ? num2[p2] - '0' : 0;
            int total = x1 + x2 + carry;
            result.Append(total % 10);
            carry = total / 10;
            p1--;
            p2--;
        }
        if (carry != 0)
            result.Append(carry);
        char[] resArray = result.ToString().ToCharArray();
        Array.Reverse(resArray);
        return new string(resArray);
    }
    
    static void Main() {
        string s1 = "0033";
        string s2 = "2";
        Console.WriteLine(multiplyStrings(s1, s2));
    }
}