Fix HashSet allowing duplicate elements

When elements are hashed to the same initial bucket, and then an element
is removed, there is a possibility that a duplicate element can be
inserted into the place of the deleted element.

Author: euchangxian

src/dataStructures/hashSet/openAddressing/HashSet.java

@@ -62,6 +62,10 @@ public HashSet() {
      * element
      */
     public boolean add(T element) {
+        if (this.contains(element)) {
+            return false;
+        }
+
         if (isLoadFactorExceeded()) {
             resize(this.capacity() * 2); // Resize to double the capacity.
         }
@@ -232,10 +236,6 @@ private int search(T element) {
             if (this.isNullBucket(currentBucketIndex)) {
                 return ELEMENT_NOT_FOUND;
             }
-            // if bucket contains TOMBSTONE, skip checking current bucket index for equality.
-            if (this.isTombstoneBucket(currentBucketIndex)) {
-                continue;
-            }
 
             // Checks equality of elements using Object::equals and Object::hashCode.
             if (this.buckets[currentBucketIndex].equals(element)