Implement Unique Quadruplets Finder (Four Sum Problem)

- Added a new class `UniqueQuadruplets` to solve the Four Sum problem, which finds all unique quadruplets in an array that sum up to a specified target value.
- Implemented the `fourSum` method that uses sorting and a two-pointer technique to efficiently identify quadruplets.
- Added example test cases in the `main` method to demonstrate functionality and validate the implementation.
- The solution ensures that duplicate quadruplets are not included in the results by skipping over duplicate values during the iteration.

Author: akarsh-Manodee

src/test/java/com/thealgorithms/misc/UniqueQuadruplets

@@ -0,0 +1,50 @@
+package com.thealgorithms.misc;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+public class UniqueQuadruplets {
+    public static List<List<Integer>> fourSum(int[] nums, int target) {
+        List<List<Integer>> result = new ArrayList<>();
+        if (nums == null || nums.length < 4) return result;
+
+        Arrays.sort(nums);
+        
+        for (int i = 0; i < nums.length - 3; i++) {
+            // Skip duplicate values
+            if (i > 0 && nums[i] == nums[i - 1]) continue;
+            for (int j = i + 1; j < nums.length - 2; j++) {
+                // Skip duplicate values
+                if (j > i + 1 && nums[j] == nums[j - 1]) continue;
+                
+                int left = j + 1, right = nums.length - 1;
+                while (left < right) {
+                    int sum = nums[i] + nums[j] + nums[left] + nums[right];
+                    if (sum == target) {
+                        result.add(Arrays.asList(nums[i], nums[j], nums[left], nums[right]));
+                        while (left < right && nums[left] == nums[left + 1]) left++; // Skip duplicates
+                        while (left < right && nums[right] == nums[right - 1]) right--; // Skip duplicates
+                        left++;
+                        right--;
+                    } else if (sum < target) {
+                        left++;
+                    } else {
+                        right--;
+                    }
+                }
+            }
+        }
+        return result;
+    }
+
+    public static void main(String[] args) {
+        int[] arr1 = {1, 0, -1, 0, -2, 2};
+        int target1 = 0;
+        System.out.println(fourSum(arr1, target1));
+
+        int[] arr2 = {4, 3, 3, 4, 4, 2, 1, 2, 1, 1};
+        int target2 = 9;
+        System.out.println(fourSum(arr2, target2));
+    }
+}