Multiply Strings

Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2.

Note:

The length of both num1 and num2 is < 110. Both num1 and num2 contains only digits 0-9. Both num1 and num2 does not contain any leading zero. You must not use any built-in BigInteger library or convert the inputs to integer directly.

Solution

public class Solution {
    public String multiply(String num1, String num2) {
          int size1 = num1.length();
        int size2 = num2.length();

        int[] result = new int[size1 * size2 + 1];

        for (int i = 0; i < size1; i++) {
            for (int j = 0; j < size2; j++) {
                int a = Integer.valueOf(String.valueOf(num1.charAt(size1 - i - 1)));
                int b = Integer.valueOf(String.valueOf((num2.charAt(size2 - j - 1))));

                result[i + j] += a * b;
            }
        }

        int[] copy = new int[size1 * size2 + 1];
        int adder = 0;

        for (int i = 0; i < size1 * size2+1; i++) {

            int sum = adder + result[i];
            copy[i] = sum % 10;
            adder = sum / 10;
        }

        StringBuffer sb = new StringBuffer();
        int start;
        for (start = size1 * size2; start >= 0; start--) {
            if (copy[start] != 0) break;
        }

        for (int i = start; i >= 0; i--) {
            sb.append(copy[i]);
        }

        if (sb.toString().equals(""))
            return "0";

        return sb.toString();
    }
}