Feat: Add FourSumProblem and JUnit Test

src/main/java/com/thealgorithms/misc/FourSumProblem.java

@@ -0,0 +1,57 @@
+package com.thealgorithms.misc;
+import java.util.*;
+
+public class FourSumProblem {
+
+    /**
+     * The function "fourSumProblem" takes an array of integers and a target integer as input, and returns a list
+     * of unique quadruplets (four elements) from the array that sum up to the target value.
+     * It avoids duplicate quadruplets by sorting the array and skipping repeated numbers.
+     * The implementation uses a two-pointer approach to efficiently find the quadruplets.
+     *
+     * @param arr An array of integers.
+     * @param target The target sum that the four integers in each quadruplet should add up to.
+     * @return A list of lists, where each inner list contains four integers that sum up to the target value.
+     * @author saivardhan (https://github.com/saivardhan15)
+     */
+
+    public static List<List<Integer>> fourSumProblem(int[] arr, int target) {
+        List<List<Integer>> ans = new ArrayList<>();
+        if (arr.length < 4) {
+            return ans;
+        }
+        // Sort the array to make it easier to detect duplicates
+        Arrays.sort(arr);
+
+        // Loop over the first two elements
+        for (int i = 0; i < arr.length - 3; i++) {
+            if (i > 0 && arr[i] == arr[i - 1]) { // Skip duplicate numbers for 'i'
+                continue;
+            }
+            for (int j = i + 1; j < arr.length - 2; j++) {
+                if (j > i + 1 && arr[j] == arr[j - 1]) { // Skip duplicate numbers for 'j'
+                    continue;
+                }
+
+                // Two-pointer technique for the remaining two elements
+                int left = j + 1;
+                int right = arr.length - 1;
+                while (left < right) {
+                    int sum = arr[i] + arr[j] + arr[left] + arr[right];
+                    if (sum == target) {
+                        ans.add(Arrays.asList(arr[i], arr[j], arr[left], arr[right]));
+                        while (left < right && arr[left] == arr[left + 1]) left++; // Skip duplicates for 'left'
+                        while (left < right && arr[right] == arr[right - 1]) right--; // Skip duplicates for 'right'
+                        left++;
+                        right--;
+                    } else if (sum < target) {
+                        left++;
+                    } else {
+                        right--;
+                    }
+                }
+            }
+        }
+        return ans;
+    }
+}

src/test/java/com/thealgorithms/misc/FourSumProblemTest.java

@@ -0,0 +1,68 @@
+package com.thealgorithms.misc;
+import static org.junit.jupiter.api.Assertions.*;
+
+import java.util.Arrays;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+public class FourSumProblemTest {
+    @Test
+    public void testEmptyArray() {
+        int[] arr = {};
+        int target = 10;
+        List<List<Integer>> result = FourSumProblem.fourSumProblem(arr, target);
+        assertTrue(result.isEmpty(), "Expected no quadruplets for an empty array");
+    }
+
+    @Test
+    public void testLessThanFourElements() {
+        int[] arr = {1, 2, 3};
+        int target = 6;
+        List<List<Integer>> result = FourSumProblem.fourSumProblem(arr, target);
+        assertTrue(result.isEmpty(), "Expected no quadruplets when array has less than 4 elements");
+    }
+
+    @Test
+    public void testNoValidQuadruplet() {
+        int[] arr = {1, 2, 3, 4, 5};
+        int target = 100;
+        List<List<Integer>> result = FourSumProblem.fourSumProblem(arr, target);
+        assertTrue(result.isEmpty(), "Expected no quadruplets when no combination matches the target");
+    }
+
+    @Test
+    public void testValidQuadruplet() {
+        int[] arr = {1, 0, -1, 0, -2, 2};
+        int target = 0;
+        List<List<Integer>> result = FourSumProblem.fourSumProblem(arr, target);
+
+        List<List<Integer>> expected = Arrays.asList(Arrays.asList(-2, -1, 1, 2), Arrays.asList(-2, 0, 0, 2), Arrays.asList(-1, 0, 0, 1));
+        assertEquals(expected.size(), result.size(), "Expected 3 valid quadruplets");
+        assertTrue(result.containsAll(expected), "The result should contain all expected quadruplets");
+    }
+
+    @Test
+    public void testWithDuplicates() {
+        int[] arr = {2, 2, 2, 2, 2};
+        int target = 8;
+        List<List<Integer>> result = FourSumProblem.fourSumProblem(arr, target);
+
+        List<List<Integer>> expected = Arrays.asList(Arrays.asList(2, 2, 2, 2));
+        assertEquals(expected.size(), result.size(), "Expected 1 valid quadruplet for repeated numbers");
+        assertTrue(result.containsAll(expected), "The result should contain the quadruplet [2, 2, 2, 2]");
+    }
+
+    @Test
+    public void testNegativeNumbers() {
+        int[] arr = {-5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5};
+        int target = 0;
+        List<List<Integer>> result = FourSumProblem.fourSumProblem(arr, target);
+
+        List<List<Integer>> expected = Arrays.asList(Arrays.asList(-5, -4, 4, 5), Arrays.asList(-5, -3, 3, 5), Arrays.asList(-5, -2, 2, 5), Arrays.asList(-5, -2, 3, 4), Arrays.asList(-5, -1, 1, 5), Arrays.asList(-5, -1, 2, 4), Arrays.asList(-5, 0, 1, 4), Arrays.asList(-5, 0, 2, 3),
+            Arrays.asList(-4, -3, 2, 5), Arrays.asList(-4, -3, 3, 4), Arrays.asList(-4, -2, 1, 5), Arrays.asList(-4, -2, 2, 4), Arrays.asList(-4, -1, 0, 5), Arrays.asList(-4, -1, 1, 4), Arrays.asList(-4, -1, 2, 3), Arrays.asList(-4, 0, 1, 3), Arrays.asList(-3, -2, 0, 5), Arrays.asList(-3, -2, 1, 4),
+            Arrays.asList(-3, -2, 2, 3), Arrays.asList(-3, -1, 0, 4), Arrays.asList(-3, -1, 1, 3), Arrays.asList(-3, 0, 1, 2), Arrays.asList(-2, -1, 0, 3), Arrays.asList(-2, -1, 1, 2));
+
+        assertEquals(expected.size(), result.size(), "Expected 24 valid quadruplets for negative and positive numbers");
+        assertTrue(result.containsAll(expected), "The result should contain all expected quadruplets");
+    }
+}