solves 4 sum

Author: anishLearnsToCode

README.md

@@ -26,6 +26,7 @@
 |  15  | [3Sum](https://leetcode.com/problems/3sum)                                                                                                                |                                                                    [![Java](assets/java.png)](src/ThreeSum.java)                                                                     |                                                                                                                                   |
 |  16  | [3Sum Closest](https://leetcode.com/problems/3sum-closest)                                                                                                |                                                                 [![Java](assets/java.png)](src/ThreeSumClosest.java)                                                                 |                                                                                                                                   |
 |  17  | [Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number)                                              |                                                        [![Java](assets/java.png)](src/LetterCombinationsOfAPhoneNumber.java)                                                         |                                                                                                                                   |
+|  18  | [4Sum](https://leetcode.com/problems/4sum)                                                                                                                |                                                                     [![Java](assets/java.png)](src/FourSum.java)                                                                     |                                                                                                                                   |
 |  20  | [ValidParentheses](https://leetcode.com/problems/valid-parentheses/)                                                                                      |                                  [![Java](assets/java.png)](src/ValidParentheses.java) [![Python](assets/python.png)](python/valid_parentheses.py)                                   |                                                                                                                                   |
 |  21  | [Merge 2 Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/)                                                                             |                                [![Java](assets/java.png)](src/Merge2SortedLists.java) [![Python](assets/python.png)](python/merge_2_sorted_lists.py)                                 |                                                                                                                                   |
 |  26  | [Remove Duplicates From Sorted Array](https://leetcode.com/problems/remove-duplicates-from-sorted-array/)                                                 |                  [![Java](assets/java.png)](src/RemoveDuplicatesFromSortedArray.java) [![Python](assets/python.png)](python/remove_duplicates_from_sorted_array.py)                  |                                                                                                                                   |

src/FourSum.java

@@ -0,0 +1,25 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+public class FourSum {
+    public List<List<Integer>> fourSum(int[] nums, int target) {
+        Arrays.sort(nums);
+        final Set<List<Integer>> quadruplets = new HashSet<>();
+
+        for (int a = 0 ; a < nums.length - 3 ; a++) {
+            for (int b = a + 1 ; b < nums.length - 2 ; b++) {
+                for (int c = b + 1, d = nums.length - 1 ; c < d ; ) {
+                    int sum = nums[a] + nums[b] + nums[c] + nums[d];
+                    if (sum == target) quadruplets.add(List.of(nums[a], nums[b], nums[c++], nums[d--]));
+                    else if (sum < target) c++;
+                    else d--;
+                }
+            }
+        }
+
+        return new ArrayList<>(quadruplets);
+    }
+}