Add comment to containsNearbyDuplicateHashSet method

Added a comment to clarify the method's purpose.

Author: harshithasudhakar

HashMap/Sliding Window/containsDuplicateHashSet.java

@@ -0,0 +1,28 @@
+class containsDuplicateHashSet {
+    public boolean containsNearbyDuplicateHashSet(int[] nums, int k) { // Optimal Solution using HashSet
+
+        HashSet<Integer> set = new HashSet<>();
+
+        for(int i = 0; i<nums.length; i++){
+            if(i >= k+1){
+                set.remove(nums[i-k-1]);
+                if(!set.contains(nums[i])){
+                    set.add(nums[i]);
+                }
+                else{
+                    return true;
+                }
+            }
+            else if(i<k+1){
+                if(!set.contains(nums[i])){
+                    set.add(nums[i]);
+                }
+                else{
+                    return true;
+                }
+            }
+        }
+
+        return false;
+    }
+}