LeetCode: Multiply Strings

public class Solution {
    public String multiply(String num1, String num2) {
        if (num1.equals("0") || num2.equals("0")) return "0";
        num1 = new StringBuilder(num1).reverse().toString();
        num2 = new StringBuilder(num2).reverse().toString();
        int[] result = new int[num1.length()+num2.length()];
        for (int i=0; i<num1.length(); i++) {
            int bit1 = num1.charAt(i)-'0';
            for (int j=0; j<num2.length(); j++) {
                result[i+j] += bit1 * (num2.charAt(j)-'0');
            }
        }
        
        String s = "";
        for (int i=0; i < result.length; i++) {
            int digit = result[i] % 10;
            int carry = result[i] / 10;
            s = Integer.toString(digit) + s;
            if (i < result.length-1)
                result[i+1] += carry;
        }
        
        int i=0;
        while(s.charAt(i) == '0') i++;
        
        return s.substring(i);
    }
}