O(n) solutions finding two elements in a series that add to a given sum

TwoSums.java

import java.util.Arrays;
import java.util.HashSet;

/**
 * Demonstrates O(n) solutions to the problem of finding two elements in a series that add to a given sum.
 * @author ncoop
 */
public class TwoSums {
    /**
     * Given an unsigned integer array <b>a</b> and a target unsigned integer <b>k</b>, returns a two-element int array
     * containing two members of a that sum to k. If no such pair exists, returns null.
     * Throws <code>IllegalArgumentException</code> if <b>k</b> is less than zero.
     *
     * Time cost: O(n), Omega(1)
     * Space cost: O(n), Omega(n)
     *
     * @param a    An array of ints; negative ints or ints greater than <b>k</b> are ignored
     * @param k    An int to which two members of a should sum
     * @return an array of two ints from <b>a</b> that sum to <b>k</b>, or null if no pair exists
     */
    public static int[] twoSumUnsigned(int[] a, int k) {
        if (k < 0 ) {
            throw new IllegalArgumentException();
        }
        boolean found[] = new boolean[k+1];
        for (int i : a) {
            if (i >= 0 && i <= k) {
                if (found[k - i]) {
                    return new int[]{i, k - i};
                }
                found[i] = true;
            }
        }
        return null;
    }

    /**
     * Given an integer array <b>a</b> and a target integer <b>k</b>, returns a two-element int array containing two
     * members of a that sum to k. If no such pair exists, returns null.
     *
     * Time cost: O(n), Omega(1)
     * Space cost: O(n), Omega(1)
     *
     * @param a    An array of ints; negative ints allowed
     * @param k    An int to which two members of a should sum
     * @return an array of two ints from <b>a</b> that sum to <b>k</b>, or null if no pair exists
     */
    public static int[] twoSum(int[] a, int k) {
        HashSet<Integer> found = new HashSet<>();
        for (int i : a) {
            if (found.contains(k-i)) {
                return new int[] {i, k-i};
            }
            found.add(i);
        }
        return null;
    }

    public static void main(String[] args) {
        Object[][] testcases = new Object[][]{
                {new int[]{0, 1, 7, 3}, 4},
                {new int[]{0, 1, 7, 3}, 7},
                {new int[]{0, 1, 7, 3}, 5},
                {new int[]{2, -2, 9, 3}, 7},
                {new int[]{2, -2, 9, 3}, 8},
                {new int[]{1, -1, 7, -3}, -4},
                {new int[]{}, 1},
                {new int[]{1}, 1},
                {/* bigArray */},
                {/* bigArray */}
        };

        int bigK = 500;
        int[] bigArray = new int[bigK/2];
        for (int i = 0; i < bigK/2; i++) {
            bigArray[i] = i+1;
        }
        testcases[testcases.length-2] = new Object[]{bigArray, bigK-1}; // big array with one solution
        testcases[testcases.length-1] = new Object[]{bigArray, bigK}; // big array with no solution

        String twoSumUnsignedSol, twoSumSol;
        System.out.println(String.format("%30s %16s %10s", "input", "twoSumUnsigned", "twoSum"));
        for (Object[] testcase : testcases) {
            try {
                twoSumUnsignedSol = Arrays.toString(twoSumUnsigned((int[]) testcase[0], (int) testcase[1]));
            } catch (IllegalArgumentException e) {
                twoSumUnsignedSol = "negative sum";
            }
            twoSumSol = Arrays.toString(twoSum((int[])testcase[0], (int)testcase[1]));
            System.out.println(String.format("%30s %16s %10s",
                    Arrays.toString((int[]) testcase[0]) + ", " + testcase[1], twoSumUnsignedSol, twoSumSol));
        }
    }
}

                         input   twoSumUnsigned     twoSum
               [0, 1, 7, 3], 4           [3, 1]     [3, 1]
               [0, 1, 7, 3], 7           [7, 0]     [7, 0]
               [0, 1, 7, 3], 5             null       null
              [2, -2, 9, 3], 7             null    [9, -2]
              [2, -2, 9, 3], 8             null       null
            [1, -1, 7, -3], -4     negative sum   [-3, -1]
                         [], 1             null       null
                        [1], 1             null       null
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250], 499       [250, 249] [250, 249]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250], 500             null       null

Process finished with exit code 0