622. 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.

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?

题目大意

设计你的循环队列实现。循环队列是一种线性数据结构，其操作表现基于 FIFO（先进先出）原则并且队尾被连接在队首之后以形成一个循环。它也被称为“环形缓冲器”。

循环队列的一个好处是我们可以利用这个队列之前用过的空间。在一个普通队列里，一旦一个队列满了，我们就不能插入下一个元素，即使在队列前面仍有空间。但是使用循环队列，我们能使用这些空间去存储新的值。

你的实现应该支持如下操作：

MyCircularQueue(k): 构造器，设置队列长度为 k 。
Front: 从队首获取元素。如果队列为空，返回 -1 。
Rear: 获取队尾元素。如果队列为空，返回 -1 。
enQueue(value): 向循环队列插入一个元素。如果成功插入则返回真。
deQueue(): 从循环队列中删除一个元素。如果成功删除则返回真。
isEmpty(): 检查循环队列是否为空。
isFull(): 检查循环队列是否已满。

解题思路

简单题。设计一个环形队列，底层用数组实现。额外维护 4 个变量，队列的总 cap，队列当前的 size，前一元素下标 left，后一个元素下标 right。每添加一个元素便维护 left，right，size，下标需要对 cap 取余，因为超过 cap 大小之后，需要循环存储。代码实现没有难度，具体sh见下面代码。

代码

package leetcode

type MyCircularQueue struct {
	cap   int
	size  int
	queue []int
	left  int
	right int
}

func Constructor(k int) MyCircularQueue {
	return MyCircularQueue{cap: k, size: 0, left: 0, right: 0, queue: make([]int, k)}
}

func (this *MyCircularQueue) EnQueue(value int) bool {
	if this.size == this.cap {
		return false
	}
	this.size++
	this.queue[this.right] = value
	this.right++
	this.right %= this.cap
	return true

}

func (this *MyCircularQueue) DeQueue() bool {
	if this.size == 0 {
		return false
	}
	this.size--
	this.left++
	this.left %= this.cap
	return true
}

func (this *MyCircularQueue) Front() int {
	if this.size == 0 {
		return -1
	}
	return this.queue[this.left]
}

func (this *MyCircularQueue) Rear() int {
	if this.size == 0 {
		return -1
	}
	if this.right == 0 {
		return this.queue[this.cap-1]
	}
	return this.queue[this.right-1]
}

func (this *MyCircularQueue) IsEmpty() bool {
	return this.size == 0
}

func (this *MyCircularQueue) IsFull() bool {
	return this.size == this.cap
}

/**
 * Your MyCircularQueue object will be instantiated and called as such:
 * obj := Constructor(k);
 * param_1 := obj.EnQueue(value);
 * param_2 := obj.DeQueue();
 * param_3 := obj.Front();
 * param_4 := obj.Rear();
 * param_5 := obj.IsEmpty();
 * param_6 := obj.IsFull();
 */