public static void printAll(int[] a) {
        int N = a.length;
        for (int i = 0; i < N; i++) {
            for (int j = i+1; j < N; j++) {
                for (int k = j+1; k < N; k++) {
                    if (a[i] + a[j] + a[k] == 0) {
                        StdOut.println(a[i] + " " + a[j] + " " + a[k]);
                    }
                }
            }
        }
    }