solves count special quadruplets

Author: anishLearnsToCode

Diff for: README.md

@@ -477,7 +477,7 @@
 | 1979 | [Find Greatest Common Divisor of Array](https://leetcode.com/problems/find-greatest-common-divisor-of-array)                                              |                                                        [![Java](assets/java.png)](src/FindGreatestCommonDivisorOfArray.java)                                                         |                                                                                                                                   |
 | 1984 | [Minimum Difference Between Highest and Lowest of K Scores](https://leetcode.com/problems/minimum-difference-between-highest-and-lowest-of-k-scores)      |                                                [![Java](assets/java.png)](src/MinimumDifferenceBetweenHighestAndLowestOfKScores.java)                                                |                                                                                                                                   |
 | 1991 | [Find the Middle Index in Array](https://leetcode.com/problems/find-the-middle-index-in-array)                                                            |                                                            [![Java](assets/java.png)](src/FindTheMiddleIndexInArray.java)                                                            |                                                                                                                                   |
-| 1995 | [Count Special Quadruplets](https://leetcode.com/problems/count-special-quadruplets)                                                                      |                                                                                                                                                                                      |                                                                                                                                   |
+| 1995 | [Count Special Quadruplets](https://leetcode.com/problems/count-special-quadruplets)                                                                      |                                                             [![Java](assets/java.png)](src/CountSpecialQuadruplets.java)                                                             |                                                                                                                                   |
 | 2000 | [Reverse Prefix of Word](https://leetcode.com/problems/reverse-prefix-of-word)                                                                            |                                                                                                                                                                                      |                                                                                                                                   |
 | 2006 | [Count Number of Pairs With Absolute Difference K](https://leetcode.com/problems/count-number-of-pairs-with-absolute-difference-k)                        |                                                                                                                                                                                      |                                                                                                                                   |
 | 2011 | [Final Value of Variable After Performing Operations](https://leetcode.com/problems/final-value-of-variable-after-performing-operations)                  |                                                                                                                                                                                      |                                                                                                                                   |

Diff for: src/CountSpecialQuadruplets.java

@@ -0,0 +1,27 @@
+// https://leetcode.com/problems/count-special-quadruplets
+// T: O(N^2)
+// S: O(N^2)
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class CountSpecialQuadruplets {
+    public int countQuadruplets(int[] nums) {
+        final Map<Integer, Integer> differenceFreq = new HashMap<>();
+        differenceFreq.put(nums[nums.length -1] - nums[nums.length - 2], 1);
+        int quadruplets = 0;
+
+        for (int b = nums.length - 3 ; b > 0 ; b--) {
+            for (int a = b - 1 ; a >= 0 ; a--) {
+                quadruplets += differenceFreq.getOrDefault(nums[a] + nums[b], 0);
+            }
+
+            for (int x = b + 1 ; x < nums.length ; x++) {
+                int difference = nums[x] - nums[b];
+                differenceFreq.put(difference, differenceFreq.getOrDefault(difference, 0) + 1);
+            }
+        }
+
+        return quadruplets;
+    }
+}