1. 题目描述

problem link

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.

2. 解题

用数组完成循环队列，由于题目要求有几个空间就要存几个数，所以不能用少存一个的方法判断队列是否满了。故再加一个字段size记录循环队列中存了几个数。

3. 代码

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef int bool;
#define true 1;
#define false 0;

typedef struct {
    int *arr;
    int size;
    int capacity;
    int head;
    int rear;
} MyCircularQueue;

/** Initialize your data structure here. Set the size of the queue to be k. */

MyCircularQueue* myCircularQueueCreate(int k) {
    MyCircularQueue* res = 
        (MyCircularQueue *)malloc(sizeof(MyCircularQueue));
    res->arr = (int *)malloc(sizeof(int) * k);
    res->size = 0;
    res->capacity = k;
    res->head = 0;
    res->rear = 0;

    return res;
}

/** Insert an element into the circular queue. Return true if the operation is successful. */
bool myCircularQueueEnQueue(MyCircularQueue* obj, int value) {
    if (obj->size == obj->capacity) {
        return false;
    }

    obj->arr[obj->rear] = value;
    obj->rear = (obj->rear + 1) % obj->capacity;
    obj->size++;
    return true;
}

/** Delete an element from the circular queue. Return true if the operation is successful. */
bool myCircularQueueDeQueue(MyCircularQueue* obj) {
    if (obj->size == 0) {
        return false;
    }

    obj->head = (obj->head + 1) % obj->capacity;
    obj->size--;
    return true;
}

/** Get the front item from the queue. */
int myCircularQueueFront(MyCircularQueue* obj) {
    if (obj->size == 0) {
        return -1;
    }
    return obj->arr[obj->head];
}

/** Get the last item from the queue. */
int myCircularQueueRear(MyCircularQueue* obj) {
    if (obj->size == 0) {
        return -1;
    }
    return obj->arr[obj->rear == 0 ? obj->capacity - 1 : obj->rear - 1];
}

/** Checks whether the circular queue is empty or not. */
bool myCircularQueueIsEmpty(MyCircularQueue* obj) {
    return obj->size == 0;
}

/** Checks whether the circular queue is full or not. */
bool myCircularQueueIsFull(MyCircularQueue* obj) {
    return obj->size == obj->capacity;
}

void myCircularQueueFree(MyCircularQueue* obj) {

}

/**
 * Your MyCircularQueue struct will be instantiated and called as such:
 * MyCircularQueue* obj = myCircularQueueCreate(k);
 * bool param_1 = myCircularQueueEnQueue(obj, value);

 * bool param_2 = myCircularQueueDeQueue(obj);

 * int param_3 = myCircularQueueFront(obj);

 * int param_4 = myCircularQueueRear(obj);

 * bool param_5 = myCircularQueueIsEmpty(obj);

 * bool param_6 = myCircularQueueIsFull(obj);

 * myCircularQueueFree(obj);
*/