0622. Design Circular Queue (M)

Design Circular Queue (M)

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:

Input
["MyCircularQueue", "enQueue", "enQueue", "enQueue", "enQueue", "Rear", "isFull", "deQueue", "enQueue", "Rear"]
[[3], [1], [2], [3], [4], [], [], [], [4], []]
Output
[null, true, true, true, false, 3, true, true, true, 4]

Explanation
MyCircularQueue myCircularQueue = new MyCircularQueue(3);
myCircularQueue.enQueue(1); // return True
myCircularQueue.enQueue(2); // return True
myCircularQueue.enQueue(3); // return True
myCircularQueue.enQueue(4); // return False
myCircularQueue.Rear();     // return 3
myCircularQueue.isFull();   // return True
myCircularQueue.deQueue();  // return True
myCircularQueue.enQueue(4); // return True
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?

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题意

实现一个循环队列。

思路

用数组很容易实现。

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代码实现

Java

class MyCircularQueue {
    private int size;
    private int head, tail;
    private int[] q;

    public MyCircularQueue(int k) {
        size = 0;
        head = tail = 0;
        q = new int[k];
    }

    public boolean enQueue(int value) {
        if (isFull()) return false;

        q[tail++] = value;
        if (tail == q.length) tail = 0;
        size++;
        return true;
    }

    public boolean deQueue() {
        if (isEmpty()) return false;

        head++;
        if (head == q.length) head = 0;
        size--;
        return true;
    }

    public int Front() {
        if (isEmpty()) return -1;
        return q[head];
    }

    public int Rear() {
        if (isEmpty()) return -1;
        int index = tail - 1 == -1 ? q.length - 1 : tail - 1;
        return q[index];
    }

    public boolean isEmpty() {
        return size == 0;
    }

    public boolean isFull() {
        return size == q.length;
    }
}