LeetCode 622. Design Circular Queue

原题链接在这里：https://leetcode.com/problems/design-circular-queue/

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.

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.

题解：

Use an array to mimic queue.

Have start to pointing to start of queue.

Have end to poining to end of queue. 

Have len to keep track of size.

When enQueue, end = (end + 1) % k, assign arr[end].

When deQueue, start = (start + 1) % k.

Rear() return arr[end].

Front() return arr[start].

Time Complexity: MyCircularQueue, O(1). endQueue, O(1). deQueue(), O(1). Front, O(1). Rear, O(1). isEmpty, O(1). isFull, O(1).

Space: O(k).

AC Java:

class MyCircularQueue {
    int [] arr;
    int k;
    int len;
    int start;
    int end;

    public MyCircularQueue(int k) {
        arr = new int[k];
        this.k = k;
        len = 0;
        start = 0;
        end = k - 1;
    }
    
    public boolean enQueue(int value) {
        if(isFull()){
            return false;
        }

        end = (end + 1) % k;
        arr[end] = value;
        len++;
        return true;
    }
    
    public boolean deQueue() {
        if(isEmpty()){
            return false;
        }

        start = (start + 1) % k;
        len--;
        return true;
    }
    
    public int Front() {
        return isEmpty() ? -1 : arr[start];
    }
    
    public int Rear() {
        return isEmpty() ? -1 : arr[end];
    }
    
    public boolean isEmpty() {
        return len == 0;
    }
    
    public boolean isFull() {
        return len == k;
    }
}

/**
 * Your MyCircularQueue object will be instantiated and called as such:
 * MyCircularQueue obj = new MyCircularQueue(k);
 * boolean param_1 = obj.enQueue(value);
 * boolean param_2 = obj.deQueue();
 * int param_3 = obj.Front();
 * int param_4 = obj.Rear();
 * boolean param_5 = obj.isEmpty();
 * boolean param_6 = obj.isFull();
 */

类似Design Circular Deque.