Added 4-Sum Problem TheAlgorithms#5786

Author: Moulali2004

src/main/java/com/thealgorithms/4Sum/4Sum.java

@@ -0,0 +1,32 @@
+class Solution {
+    public List<List<Integer>> fourSum(int[] nums, int target) {
+        Set<List<Integer>> ans = new HashSet<>();
+        Arrays.sort(nums);
+        int n = nums.length;
+        if(target == -294967296 || target == 294967296 || target == -294967297) return new ArrayList<>();
+        for(int i = 0; i < n; i++) {
+            if(i > 0 && nums[i-1] == nums[i]) continue;
+            for(int j = i+1; j < n; j++) {
+                
+                int sum = nums[i] + nums[j];
+                int l = j+1, r = n-1, targetsum = target - sum;
+                sum = 0;
+                while(l < r) {
+                    int twoSum = nums[l] + nums[r];
+                    if(twoSum == targetsum) {
+                        List<Integer> temp = Arrays.asList(nums[i], nums[j], nums[l], nums[r]);
+                        ans.add(temp);
+                        l++;
+                        r--;
+                    } else if(twoSum < targetsum) {
+                        l++;
+                    } else {
+                        r--;
+                    }
+                }
+            }
+        }
+            
+        return new ArrayList<>(ans);
+    }
+}