LeetCode: Multiply Strings

LeetCode: Multiply Strings 

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/**
 * Description: Given two numbers represented as strings, return multiplication of the numbers as a string.
 * <p/>
 * Note: The numbers can be arbitrarily large and are non-negative.
 *
 * @author hzhou
 */
public class MultiplyStrings {
    public String multiply(String num1, String num2) {
        if (num1 == null || num1.isEmpty() || num2 == null || num2.isEmpty() || num1.equals("0") || num2.equals("0")) {
            return "0";
        }
        int[] tmp = new int[num1.length() + num2.length()];
        String t1 = new StringBuilder(num1).reverse().toString();
        String t2 = new StringBuilder(num2).reverse().toString();
        for (int i = 0; i < t1.length(); i++) {
            for (int j = 0; j < t2.length(); j++) {
                tmp[i + j] += calc(t1.charAt(i), t2.charAt(j));
            }
        }
        int inc = 0;
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < tmp.length; i++) {
            int sum = inc + tmp[i];
            inc = sum / 10;
            tmp[i] = sum % 10;
            sb.append(tmp[i]);
        }
        String result = sb.reverse().toString();
        int i = 0;
        while (result.charAt(i) == '0') {
            i++;
        }
        return result.substring(i, result.length());
    }
    private int calc(char a, char b) {
        return (a - '0') * (b - '0');
    }
    @Test
    public void test() {
        String r = multiply("0", "0");
        System.out.println(r);
    }
}