621 Task Scheduler 任务调度器

Description:
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:

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?

题目描述:
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。

示例 :

MyCircularQueue circularQueue = new MyCircularQueue(3); // 设置长度为 3
circularQueue.enQueue(1);  // 返回 true
circularQueue.enQueue(2);  // 返回 true
circularQueue.enQueue(3);  // 返回 true
circularQueue.enQueue(4);  // 返回 false，队列已满
circularQueue.Rear();  // 返回 3
circularQueue.isFull();  // 返回 true
circularQueue.deQueue();  // 返回 true
circularQueue.enQueue(4);  // 返回 true
circularQueue.Rear();  // 返回 4

提示:

所有的值都在 0 至 1000 的范围内；
操作数将在 1 至 1000 的范围内；
请不要使用内置的队列库。

思路:

用一个数组记录数据
用两个指针分别指向队列的第一个元素和最后一个元素后面的空位
额外增加一个空位或者记录数组中的有效元素个数来判断数组是否已满
时间复杂度 O(1), 空间复杂度 O(n)

代码:
C++:

class MyCircularQueue 
{
private:
    int count = 0, head = 0, rear = 0, *data = nullptr, capacity = 0;
public:
    MyCircularQueue(int k) 
    {
        data = new int[k];
        capacity = k;
    }
    
    bool enQueue(int value) 
    {
        if (isFull()) return false;
        data[rear] = value;
        rear = (rear + 1) % capacity;
        ++count;
        return true;
    }
    
    bool deQueue() 
    {
        if (isEmpty()) return false;
        head = (head + 1) % capacity;
        --count;
        return true;
    }
    
    int Front() 
    {
        return isEmpty() ? -1 : data[head];
    }
    
    int Rear() 
    {
        return isEmpty() ? -1 : (rear == 0 ? data[capacity - 1] : data[rear - 1]);
    }
    
    bool isEmpty() 
    {
        return count == 0;
    }
    
    bool isFull() 
    {
        return count == capacity;
    }
};

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

Java:

class MyCircularQueue {
    int[] queue;
    int count, front, rear;

    /** Initialize your data structure here. Set the size of the queue to be k. */
    public MyCircularQueue(int k) {
        queue = new int[k];
    }
    
    /** Insert an element into the circular queue. Return true if the operation is successful. */
    public boolean enQueue(int value) {
        if (isFull()) return false;
        queue[rear] = value;
        rear = (rear + 1) % queue.length;
        ++count;
        return true;
    }
    
    /** Delete an element from the circular queue. Return true if the operation is successful. */
    public boolean deQueue() {
        if (isEmpty()) return false;
        front = (front + 1) % queue.length;
        --count;
        return true;
    }
    
    /** Get the front item from the queue. */
    public int Front() {
        if (isEmpty()) return -1;
        return queue[front];
    }
    
    /** Get the last item from the queue. */
    public int Rear() {
        if (isEmpty()) return -1;
        return rear == 0 ? queue[queue.length - 1] : queue[rear - 1];
    }
    
    /** Checks whether the circular queue is empty or not. */
    public boolean isEmpty() {
        return count == 0;
    }
    
    /** Checks whether the circular queue is full or not. */
    public boolean isFull() {
        return count == queue.length;
    }
}

/**
 * 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();
 */

Python:

class MyCircularQueue:

    def __init__(self, k: int):
        self.size = k + 1
        self.data = [0] * (k + 1)
        self.head = self.rear = 0

        
    def enQueue(self, value: int) -> bool:
        if self.isFull():
            return False
        self.data[self.rear] = value
        self.rear = (self.rear + 1) % self.size
        return True

    
    def deQueue(self) -> bool:
        if self.isEmpty():
            return False
        self.head = (self.head + 1) % self.size
        return True


    def Front(self) -> int:
        return -1 if self.isEmpty() else self.data[self.head]
    

    def Rear(self) -> int:
        return -1 if self.isEmpty() else self.data[(self.rear - 1) % self.size]


    def isEmpty(self) -> bool:
        return self.head == self.rear


    def isFull(self) -> bool:
        return (self.head - self.rear) % self.size == 1



# Your MyCircularQueue object will be instantiated and called as such:
# obj = MyCircularQueue(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()