Added four-sum problem

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

@@ -0,0 +1,57 @@
+package com.thealgorithms.misc;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import org.apache.commons.lang3.tuple.Pair;
+/**
+ * Four Sum Problem
+ * 
+ * This algorithm finds all unique quadruplets in an array that sum up to a given target.
+ * 
+ * URL: https://en.wikipedia.org/wiki/Subset_sum
+ */
+public class FourSumProblem {
+
+    public static List<Pair<Integer, Integer>> fourSum(int[] values, int target) {
+        List<Pair<Integer, Integer>> result = new ArrayList<>();
+        if (values == null || values.length < 4) {
+            return result;
+        }
+
+        Arrays.sort(values);
+
+        for (int i = 0; i < values.length - 3; i++) {
+            if (i > 0 && values[i] == values[i - 1]) {
+                continue; // Skip duplicates
+            }
+            for (int j = i + 1; j < values.length - 2; j++) {
+                if (j > i + 1 && values[j] == values[j - 1]) {
+                    continue; // Skip duplicates
+                }
+
+                int left = j + 1;
+                int right = values.length - 1;
+
+                while (left < right) {
+                    int sum = values[i] + values[j] + values[left] + values[right];
+                    if (sum == target) {
+                        result.add(Pair.of(left, right));
+
+                        // Skip duplicates
+                        while (left < right && values[left] == values[left + 1]) left++;
+                        while (left < right && values[right] == values[right - 1]) right--;
+
+                        left++;
+                        right--;
+                    } else if (sum < target) {
+                        left++;
+                    } else {
+                        right--;
+                    }
+                }
+            }
+        }
+        return result;
+    }
+}

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

@@ -0,0 +1,62 @@
+package com.thealgorithms.misc;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+
+import org.apache.commons.lang3.tuple.Pair;
+import org.junit.jupiter.api.Test;
+import java.util.List;
+
+/**
+ * Test case for Four sum Problem.
+ */
+public class FourSumProblemTest {
+
+    @Test
+    void testFourSumExists() {
+        final int[] values = new int[] {1, 0, -1, 0, -2, 2};
+        final int target = 0;
+        // Expecting one solution (e.g., indices of the numbers summing to target)
+        final var expected = List.of(
+                Pair.of(0, 5), // -2 + 2 + 1 + -1 = 0
+                Pair.of(1, 4)  // 0 + 0 + -1 + 1 = 0
+        );
+        assertEquals(expected, FourSumProblem.fourSum(values, target));
+    }
+
+    @Test
+    void testFourSumNoSolution() {
+        final int[] values = new int[] {1, 2, 3, 4};
+        final int target = 100;
+        assertFalse(FourSumProblem.fourSum(values, target).isEmpty());
+    }
+
+    @Test
+    void testFourSumMultipleSolutions() {
+        final int[] values = new int[] {1, 0, -1, 0, -2, 2};
+        final int target = 0;
+        final var expected = List.of(
+                Pair.of(0, 5), // -2 + 2 + 1 + -1 = 0
+                Pair.of(1, 4)  // 0 + 0 + -1 + 1 = 0
+        );
+        assertEquals(expected, FourSumProblem.fourSum(values, target));
+    }
+
+    @Test
+    void testFourSumNegativeNumbers() {
+        final int[] values = new int[] {-1, -2, -3, -4, -5, 5};
+        final int target = -10;
+        final var expected = List.of(
+                Pair.of(1, 3), // -2 + -3 + -5 + 5 = -10
+                Pair.of(0, 4)  // -1 + -4 + -5 + 5 = -10
+        );
+        assertEquals(expected, FourSumProblem.fourSum(values, target));
+    }
+
+    @Test
+    void testFourSumNoSolutionDuplicatedInputs() {
+        final int[] values = new int[] {1, 1, 1, 1};
+        final int target = 10;
+        assertFalse(FourSumProblem.fourSum(values, target).isEmpty());
+    }
+}