题目

Design your implementation of the circular queue. The circular queue is a linear data structure in which the operations are performed based on FIFO (First In First Out) principle and the last position is connected back to the first position to make a circle. It is also called “Ring Buffer”.

One of the benefits of the circular queue is that we can make use of the spaces in front of the queue. In a normal queue, once the queue becomes full, we cannot insert the next element even if there is a space in front of the queue. But using the circular queue, we can use the space to store new values.

Implementation the MyCircularQueue class:

  • MyCircularQueue(k) Initializes the object with the size of the queue to be k.
  • int Front() Gets the front item from the queue. If the queue is empty, return -1.
  • int Rear() Gets the last item from the queue. If the queue is empty, return -1.
  • boolean enQueue(int value) Inserts an element into the circular queue. Return true if the operation is successful.
  • boolean deQueue() Deletes an element from the circular queue. Return true if the operation is successful.
  • boolean isEmpty() Checks whether the circular queue is empty or not.
  • boolean isFull() Checks whether the circular queue is full or not.

Example 1:

  1. Input
  2. ["MyCircularQueue", "enQueue", "enQueue", "enQueue", "enQueue", "Rear", "isFull", "deQueue", "enQueue", "Rear"]
  3. [[3], [1], [2], [3], [4], [], [], [], [4], []]
  4. Output
  5. [null, true, true, true, false, 3, true, true, true, 4]
  7. Explanation
  8. MyCircularQueue myCircularQueue = new MyCircularQueue(3);
  9. myCircularQueue.enQueue(1); // return True
  10. myCircularQueue.enQueue(2); // return True
  11. myCircularQueue.enQueue(3); // return True
  12. myCircularQueue.enQueue(4); // return False
  13. myCircularQueue.Rear();     // return 3
  14. myCircularQueue.isFull();   // return True
  15. myCircularQueue.deQueue();  // return True
  16. myCircularQueue.enQueue(4); // return True
  17. myCircularQueue.Rear();     // return 4

Constraints:

  • 1 <= k <= 1000
  • 0 <= value <= 1000
  • At most 3000 calls will be made to enQueue, deQueue, Front, Rear, isEmpty, and isFull.

Follow up: Could you solve the problem without using the built-in queue?

题意

实现一个循环队列。

思路

用数组很容易实现。

代码实现

Java

  1. class MyCircularQueue {
  2.     private int size;
  3.     private int head, tail;
  4.     private int[] q;
  6.     public MyCircularQueue(int k) {
  7.         size = 0;
  8.         head = tail = 0;
  9.         q = new int[k];
  10.     }
  12.     public boolean enQueue(int value) {
  13.         if (isFull()) return false;
  15.         q[tail++] = value;
  16.         if (tail == q.length) tail = 0;
  17.         size++;
  18.         return true;
  19.     }
  21.     public boolean deQueue() {
  22.         if (isEmpty()) return false;
  24.         head++;
  25.         if (head == q.length) head = 0;
  26.         size--;
  27.         return true;
  28.     }
  30.     public int Front() {
  31.         if (isEmpty()) return -1;
  32.         return q[head];
  33.     }
  35.     public int Rear() {
  36.         if (isEmpty()) return -1;
  37.         int index = tail - 1 == -1 ? q.length - 1 : tail - 1;
  38.         return q[index];
  39.     }
  41.     public boolean isEmpty() {
  42.         return size == 0;
  43.     }
  45.     public boolean isFull() {
  46.         return size == q.length;
  47.     }
  48. }