Design HashSet

Design a HashSet without using any built-in hash table libraries.

To be specific, your design should include these functions:

• add(value): Insert a value into the HashSet.
• contains(value) : Return whether the value exists in the HashSet or not.
• remove(value): Remove a value in the HashSet. If the value does not exist in the HashSet, do nothing.

Example:

MyHashSet hashSet = new MyHashSet();
hashSet.add(1);        
hashSet.add(2);        
hashSet.contains(1);    // returns true
hashSet.contains(3);    // returns false (not found)
hashSet.add(2);          
hashSet.contains(2);    // returns true
hashSet.remove(2);          
hashSet.contains(2);    // returns false (already removed)

 class MyHashSet {
     final ListNode[] nodes = new ListNode[];
     /** Initialize your data structure here. */
     public MyHashSet() {

     }

     public void add(int key) {
         int i = idx(key);
         ListNode first = nodes[i];
         if (first == null) {
             nodes[i] = new ListNode(key);
         } else if (!exists(first, key)) {
             ListNode newNode = new ListNode(key);
             newNode.next = nodes[i];
             nodes[i] = newNode;
         }
     }

     public void remove(int key) {
         int i = idx(key);
         if (nodes[i] == null) {
             return;
         }
         ListNode current = nodes[i];
         ListNode previous = null;
         while (current != null) {
             if (current.key == key) {
                 if (previous != null) {
                     previous.next = current.next;
                 } else {
                     nodes[i] = current.next;
                 }
                 break;
             } else {
                 previous = current;
                 current = current.next;
             }
         }
     }

     /** Returns true if this set contains the specified element */
     public boolean contains(int key) {
         int i = idx(key);
         ListNode first = nodes[i];
         if (first == null) {
             return false;
         }
         return exists(first, key);
     }

     int idx(int key) {
         return key % nodes.length;
     }

     private boolean exists(ListNode node, int key) {
         ListNode current = node;
         while (current != null) {
             if (current.key == key) {
                 return true;
             }
             current = current.next;
         }
         return false;
     }
 }

 class ListNode {
     int key;
     ListNode next;

     ListNode(int key) {
         this.key = key;
     }
 }