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    // Complexity - O(n^4)
    private int[] findQuad_1(int[] A, int[] B, int[] C, int[] D) {
        for (int i = 0, al = A.length; i < al; i++)
            for (int j = 0, bl = B.length; j < bl; j++)
                for (int k = 0, cl = C.length; k < cl; k++)
                    for (int l = 0, dl = D.length; l < dl; l++)
                        if (A[i] + B[j] + C[k] + D[l] == 0) return new int[]{A[i], B[j], C[k], D[l]};
        return null;
    }