MultiplyStringsM.java

//Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string.
//
//        Note: You must not use any built-in BigInteger library or convert the inputs to integer directly.

public class MultiplyStringsM {

    public static void main(String[] args) {
        String num1 = "12";
        String res = multiply(num1,num1);
        System.out.println(res);
    }

    public static String multiply(String num1, String num2) {
        int m = num1.length(), n = num2.length();
        int[] pos = new int[m + n];

        for(int i = m - 1; i >= 0; i--) {
            for(int j = n - 1; j >= 0; j--) {
                int mul = (num1.charAt(i) - '0') * (num2.charAt(j) - '0');
                int p1 = i + j, p2 = i + j + 1;
                int sum = mul + pos[p2];

                pos[p1] += sum / 10;
                pos[p2] = (sum) % 10;
            }
        }

        StringBuilder sb = new StringBuilder();
        for(int p : pos) if(!(sb.length() == 0 && p == 0)) sb.append(p);
        return sb.length() == 0 ? "0" : sb.toString();
    }
}
//Funny problem, rules is you aren't allowed to change strings directly into integers to multiply