Use LinkedList to implement Queue
1 /* H tail 2 (last in) 1->2->3->4 (1st in) 3 o/H 4 first-out 5 6 */ 7 public class UseLinkedListImplementQueue { 8 private ListNode head ; 9 private ListNode tail ; 10 private int length; 11 12 public UseLinkedListImplementQueue() { 13 head = null ; 14 tail = null ; 15 length = 0; 16 } 17 18 public void offer(int val){ 19 ListNode node = new ListNode(val) ; 20 node.next = head ; 21 head = node ; 22 length++; 23 syncHeadAndTail(); 24 } 25 // 1->2->3->4 to 1->2->3 26 public Integer poll(){ 27 if (tail == null) return null; 28 int value = tail.val ; 29 ListNode curr = head ; 30 while (curr != null && curr.next != tail){ 31 curr = curr.next ; 32 } 33 //now curr.next -> tail 34 curr.next = null ; 35 //move tail to curr 36 tail = curr ; 37 length-- ; 38 syncHeadAndTail(); 39 return value ; 40 } 41 42 public Integer peek(){ 43 if (tail == null) return null ; 44 return tail.val ; 45 } 46 47 public boolean isEmpty(){ 48 return this.length <= 0 ; 49 } 50 51 public int getSize(){ 52 return this.length ; 53 } 54 55 private void syncHeadAndTail(){ 56 if (this.length == 0){ 57 head = null ; 58 tail = null ; 59 } 60 if (this.length == 1){ 61 tail = head ; 62 } 63 } 64 65 public static void main(String[] args) { 66 UseLinkedListImplementQueue queue = new UseLinkedListImplementQueue() ; 67 queue.offer(1); 68 queue.offer(2); 69 System.out.println(queue.poll()); 70 queue.offer(3); 71 queue.offer(4); 72 System.out.println(queue.poll()); 73 System.out.println(queue.poll()); 74 75 }