43 Multiply Strings

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

Example 1:

Input: num1 = "2", num2 = "3"
Output: "6"
Example 2:

Input: num1 = "123", num2 = "456"
Output: "56088"
Note:

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

class Solution {
    public String multiply(String num1, String num2) {
         if (num1 == null || num2 == null) return "0";
        int[] digits = new int[num1.length() + num2.length()];
        for (int i = num1.length() - 1; i >= 0; i--) {
            for (int j = num2.length() - 1; j >= 0; j--) {
                int product = (num1.charAt(i) - '0') * (num2.charAt(j) - '0');
                int p1 = i + j, p2 = i + j + 1;
                int sum = product + digits[p2];
                digits[p1] += sum / 10;
                digits[p2] = sum % 10;
            }
        }
        StringBuilder res = new StringBuilder();
        for (int digit : digits) {
            if (!(digit == 0 && res.length() == 0)) {
                res.append(digit);
            }
        }
        return res.length() == 0 ? "0" : res.toString();
    }
}