目录

一. 概念

二. 队列的使用

 三. 队列模拟实现

四. 循环队列

五. 面试题

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一. 概念

在Queue接口的方法里, 我们看到有add remove element 这些是继承于Collection接口的, 而offer poll peek是接口本身的,他们的使用效果基本一致

二. 队列的使用

注意：Queue是个接口，在实例化时必须实例化LinkedList的对象，因为LinkedList实现了Queue接口。 

public static void main ( String [] args ) {

        Queue < Integer > q = new LinkedList <> ();

        q . offer ( 1 );

        q . offer ( 2 );

        q . offer ( 3 );

        q . offer ( 4 );

        q . offer ( 5 ); // 从队尾入队列

        System . out . println ( q . size ());

        System . out . println ( q . peek ()); // 获取队头元素

        q . poll ();

        System . out . println ( q . poll ()); // 从队头出队列，并将删除的元素返回

        if ( q . isEmpty ()){

                System . out . println ( " 队列空 " );

        } else {

                System . out . println ( q . size ());

        }

}

 三. 队列模拟实现

数组下标循环的小技巧

 

队尾指针向后:

rear = (rear + 偏移量) %array.length

队尾指针向前:

rear = (rear + array.length - 偏移量) %array.length

设计一个循环队列 链接

class MyCircularQueue {public int[] elem;public int front;public int rear;public MyCircularQueue(int k) {elem = new int[k+1];}//入队public boolean enQueue(int value) {if(isFull()){return false;}elem[rear] = value;rear = (rear+1)%elem.length;return true;}//删除队头元素public boolean deQueue() {if(isEmpty()){return false;}front = (front + 1)%elem.length;return true;}//得到队头元素public int Front() {if(isEmpty()){return -1;}return elem[front];}//得到队尾元素public int Rear() {if(isEmpty()){return -1;}int index = (rear + elem.length - 1)%elem.length;return elem[index];}public boolean isEmpty() {return front == rear;}public boolean isFull() {return (rear + 1)%elem.length == front;}
}

五. 双端队列

 Deque是一个接口，使用时必须创建LinkedList的对象。

Deque<Integer> stack = new ArrayDeque<>();// 双端队列的线性实现

Deque<Integer> queue = new LinkedList<>();// 双端队列的链式实现

五. 面试题

1. 用队列实现栈 链接

思路:一个队列是不能实现栈的, 需要用两个队列

1. 入栈, 当两个队列都为空时, 放在第一个队列里

2. 再次入栈时, 放在不为空的队列

3. 出栈时, 出不为空的队列, 出size-1个元素, 剩下的一个就是要出栈的元素

代码:

class MyStack {private Queue<Integer> qu1;private Queue<Integer> qu2;public MyStack() {qu1 = new LinkedList<>();qu2 = new LinkedList<>();}public void push(int x) {if(empty()){qu1.offer(x);return;}if(!qu1.isEmpty()){qu1.offer(x);}else{qu2.offer(x);}}public int pop() {if(empty()){return -1;}if(!qu1.isEmpty()){int size = qu1.size();for(int i = 0;i < size-1;i++){qu2.offer(qu1.poll());}return qu1.poll();}else{int size = qu2.size();for(int i = 0;i < size-1;i++){qu1.offer(qu2.poll());}return qu2.poll();}}public int top() {if(empty()){return -1;}if(!qu1.isEmpty()){int size = qu1.size();for(int i = 0;i < size-1;i++){qu2.offer(qu1.poll());}int tmp = qu1.peek();qu2.offer(qu1.poll());return tmp;}else{int size = qu2.size();for(int i = 0;i < size-1;i++){qu1.offer(qu2.poll());}int tmp = qu2.peek();qu1.offer(qu2.poll());return tmp;}}public boolean empty() {return qu1.isEmpty() && qu2.isEmpty();}
}

2. 用栈实现队列 链接

思路:一个栈不能实现队列, 需要两个栈

1. 入队, 把数据放在第一个栈

2. 出栈, 出第二个栈的栈顶元素即可, 如果第二个栈为空, 将里面的所有元素放到第二个栈

3. 当两个栈都为空时, 说明模拟的队列为空

class MyQueue {private Stack<Integer> s1;private Stack<Integer> s2;public MyQueue() {s1 = new Stack<>();s2 = new Stack<>();}public void push(int x) {s1.push(x);}public int pop() {if(empty()){return -1;}if(s2.isEmpty()){while(!s1.isEmpty()){s2.push(s1.pop());}}return s2.pop();}public int peek() {if(empty()){return -1;}if(s2.isEmpty()){while(!s1.isEmpty()){s2.push(s1.pop());}}return s2.peek();}public boolean empty() {return s1.isEmpty() && s2.isEmpty();}
}