leet43：字符串相乘

public static String multiply(String num1, String num2) {
    if (num1.equals("0") || num2.equals("0")) {
        return "0";
    }
    int[] res = new int[num1.length() + num2.length()];
    for (int i = num1.length() - 1; i >= 0; i--) {
        int n1 = num1.charAt(i) - '0';
        for (int j = num2.length() - 1; j >= 0; j--) {
            int n2 = num2.charAt(j) - '0';
            int sum = (res[i + j + 1] + n1 * n2);
            res[i + j + 1] = sum % 10;
            res[i + j] += sum / 10;
        }
    }
    StringBuilder result = new StringBuilder();
    for (int i = 0; i < res.length; i++) {
        if (i == 0 && res[i] == 0) continue;
        result.append(res[i]);
    }
    return result.toString();
}