Use LinkedList to implement Queue

/*             H            tail
         (last in) 1->2->3->4 (1st in)
                      o/H
                            first-out

     */
public class UseLinkedListImplementQueue {
    private ListNode head ;
    private ListNode tail ;
    private int length;

    public UseLinkedListImplementQueue() {
        head = null ;
        tail = null ;
        length = 0;
    }

    public void offer(int val){
        ListNode node = new ListNode(val) ;
        node.next = head ;
        head = node ;
        length++;
        syncHeadAndTail();
    }
    // 1->2->3->4   to 1->2->3
    public Integer poll(){
        if (tail == null)  return null;
        int value = tail.val ;
        ListNode curr = head ;
        while (curr != null && curr.next != tail){
            curr = curr.next ;
        }
        //now curr.next -> tail
        curr.next = null ;
        //move tail to curr
        tail = curr ;
        length-- ;
        syncHeadAndTail();
        return value ;
    }

    public Integer peek(){
        if (tail == null) return null ;
        return tail.val ;
    }

    public boolean isEmpty(){
        return this.length <= 0 ;
    }

    public int getSize(){
        return this.length ;
    }

    private void syncHeadAndTail(){
     if (this.length == 0){
         head = null ;
         tail = null ;
     }
     if (this.length == 1){
         tail = head ;
     }
    }

    public static void main(String[] args) {
        UseLinkedListImplementQueue queue = new UseLinkedListImplementQueue() ;
        queue.offer(1);
        queue.offer(2);
        System.out.println(queue.poll());
        queue.offer(3);
        queue.offer(4);
        System.out.println(queue.poll());
        System.out.println(queue.poll());

    }