Created FourSumProblem

Author: Krushit-Babariya

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

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+package com.thealgorithms.misc;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+public class FourSumProblem {
+    public List<List<Integer>> fourSum(int[] nums, int target) {
+        int n = nums.length;
+        List<List<Integer>> ans = new ArrayList<>();
+        Arrays.sort(nums);
+
+        for (int i = 0; i < n; i++) {
+            if (i > 0 && nums[i] == nums[i - 1]) {
+                continue;
+            }
+
+            for (int j = i + 1; j < n; j++) {
+                if (j > i + 1 && nums[j] == nums[j - 1]) {
+                    continue;
+                }
+
+                int k = j + 1;
+                int l = n - 1;
+
+                while (k < l) {
+                    long sum = (long) nums[i] + nums[j] + nums[k] + nums[l];
+
+                    if (sum == target) {
+                        ans.add(Arrays.asList(nums[i], nums[j], nums[k], nums[l]));
+                        k++;
+                        l--;
+
+                        while (k < l && nums[k] == nums[k - 1]) {
+                            k++;
+                        }
+                        while (k < l && nums[l] == nums[l + 1]) {
+                            l--;
+                        }
+                    } else if (sum < target) {
+                        k++;
+                    } else {
+                        l--;
+                    }
+                }
+            }
+        }
+        return ans;
+    }
+}

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

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+package com.thealgorithms.misc;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.*;
+
+import java.util.Arrays;
+import java.util.List;
+
+public class FourSumProblemTest {
+    
+    private final FourSumProblem fourSumProblem = new FourSumProblem();
+
+    @Test
+    public void testFourSum_WithValidInput() {
+        int[] nums = {1, 0, -1, 0, -2, 2};
+        int target = 0;
+        List<List<Integer>> expected = Arrays.asList(
+            Arrays.asList(-2, 0, 0, 2),
+            Arrays.asList(-1, 0, 0, 1)
+        );
+
+        List<List<Integer>> result = fourSumProblem.fourSum(nums, target);
+        assertEquals(expected.size(), result.size());
+        assertTrue(result.containsAll(expected));
+    }
+
+    @Test
+    public void testFourSum_WithNoSolution() {
+        int[] nums = {1, 2, 3, 4};
+        int target = 100;
+        List<List<Integer>> result = fourSumProblem.fourSum(nums, target);
+        assertTrue(result.isEmpty());
+    }
+
+    @Test
+    public void testFourSum_WithDuplicates() {
+        int[] nums = {1, 1, 1, 1};
+        int target = 4;
+        List<List<Integer>> expected = Arrays.asList(
+            Arrays.asList(1, 1, 1, 1)
+        );
+
+        List<List<Integer>> result = fourSumProblem.fourSum(nums, target);
+        assertEquals(expected.size(), result.size());
+        assertTrue(result.containsAll(expected));
+    }
+
+    @Test
+    public void testFourSum_WithNegativeNumbers() {
+        int[] nums = {-3, -2, -1, 0, 0, 1, 2, 3};
+        int target = 0;
+        List<List<Integer>> expected = Arrays.asList(
+            Arrays.asList(-3, 1, 1, 1),
+            Arrays.asList(-2, 0, 0, 2),
+            Arrays.asList(-1, 0, 0, 1)
+        );
+
+        List<List<Integer>> result = fourSumProblem.fourSum(nums, target);
+        assertEquals(expected.size(), result.size());
+        assertTrue(result.containsAll(expected));
+    }
+
+    @Test
+    public void testFourSum_WithEmptyArray() {
+        int[] nums = {};
+        int target = 0;
+        List<List<Integer>> result = fourSumProblem.fourSum(nums, target);
+        assertTrue(result.isEmpty());
+    }
+
+    @Test
+    public void testFourSum_WithLessThanFourElements() {
+        int[] nums = {1, 2, 3};
+        int target = 6;
+        List<List<Integer>> result = fourSumProblem.fourSum(nums, target);
+        assertTrue(result.isEmpty());
+    }
+}