43. Multiply Strings

class Solution {
    public String multiply(String num1, String num2) {
        int m = num1.length(), n = num2.length();
        int[] res = 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 + res[p2];
                res[p1] += sum / 10;  // res[p1] can be larger than 9
                res[p2] = sum % 10;
            }
        }

        StringBuilder sb = new StringBuilder();
        for (int p: res) {
            if (p == 0 && sb.length() == 0) continue;
            sb.append(p);
        }

        return sb.length() == 0 ? "0" : sb.toString();
    }
}