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.

Your implementation should support following operations:

• MyCircularQueue(k): Constructor, set the size of the queue to be k.
• Front: Get the front item from the queue. If the queue is empty, return -1.
• Rear: Get the last item from the queue. If the queue is empty, return -1.
• enQueue(value): Insert an element into the circular queue. Return true if the operation is successful.
• deQueue(): Delete an element from the circular queue. Return true if the operation is successful.
• isEmpty(): Checks whether the circular queue is empty or not.
• isFull(): Checks whether the circular queue is full or not.

Example:

MyCircularQueue circularQueue = new MyCircularQueue(3); // set the size to be 3
circularQueue.enQueue(1); // return true
circularQueue.enQueue(2); // return true
circularQueue.enQueue(3); // return true
circularQueue.enQueue(4); // return false, the queue is full
circularQueue.Rear(); // return 3
circularQueue.isFull(); // return true
circularQueue.deQueue(); // return true
circularQueue.enQueue(4); // return true
circularQueue.Rear(); // return 4

Note:

• All values will be in the range of [0, 1000].
• The number of operations will be in the range of[1, 1000].
• Please do not use the built-in Queue library.

設計迴圈佇列。

題目就是題意。這是一道設計題，有幾個函式需要實現。

• MyCircularQueue(k) - 一個長度為K的queue
• Front: 讀取佇列頭部的元素，如果佇列為空則返回-1
• Rear: 讀取佇列尾部的元素，如果佇列為空則返回-1
• enQueue(value):從佇列的頭部加入一個元素，如果能加入return true，不能return false
• deQueue():從佇列的尾部加入一個元素，如果能加入return true，不能return false
• isEmpty(): 判斷佇列是否為空
• isFull(): 判斷佇列是否滿了

思路還是比較直觀的，既然是環形佇列，說明一定是隻能使用固定大小的記憶體。為了達到這個題的練習目的，這道題我用陣列實現。建立一個長度為K的陣列，同時建立幾個變數，front, rear是陣列前後的兩個指標，len記錄陣列的長度。

時間O(n)

空間O(n)

Java實現

 1 class MyCircularQueue {
 2     final int[] a;
 3     int front, rear = -1, len = 0;
 4 
 5     public MyCircularQueue(int k) {
 6         a = new int[k];
 7     }
 8 
 9     public boolean enQueue(int val) {
10         if (!isFull()) {
11             rear = (rear + 1) % a.length;
12             a[rear] = val;
13             len++;
14             return true;
15         } else
16             return false;
17     }
18 
19     public boolean deQueue() {
20         if (!isEmpty()) {
21             front = (front + 1) % a.length;
22             len--;
23             return true;
24         } else
25             return false;
26     }
27 
28     public int Front() {
29         return isEmpty() ? -1 : a[front];
30     }
31 
32     public int Rear() {
33         return isEmpty() ? -1 : a[rear];
34     }
35 
36     public boolean isEmpty() {
37         return len == 0;
38     }
39 
40     public boolean isFull() {
41         return len == a.length;
42     }
43 }
44 
45 /**
46  * Your MyCircularQueue object will be instantiated and called as such:
47  * MyCircularQueue obj = new MyCircularQueue(k);
48  * boolean param_1 = obj.enQueue(value);
49  * boolean param_2 = obj.deQueue();
50  * int param_3 = obj.Front();
51  * int param_4 = obj.Rear();
52  * boolean param_5 = obj.isEmpty();
53  * boolean param_6 = obj.isFull();
54  */

LeetCode 題目總結