Java中 Queue队列的用法及例子

1. Usage 用法

// "static void main" must be defined in a public class.
public class Main {
    public static void main(String[] args) {
        // 1. Initialize a queue.
        Queue<Integer> q = new LinkedList();
        // 2. Get the first element - return null if queue is empty.
        System.out.println("The first element is: " + q.peek());
        // 3. Push new element.
        q.offer(5);
        q.offer(13);
        q.offer(8);
        q.offer(6);
        // 4. Pop an element.
        q.poll();
        // 5. Get the first element.
        System.out.println("The first element is: " + q.peek());
        // 7. Get the size of the queue.
        System.out.println("The size is: " + q.size());
    }
}

例 Moving Average from Data Stream

Given a stream of integers and a window size, calculate the moving average of all integers in the sliding window.

Example:

MovingAverage m = new MovingAverage(3);
m.next(1) = 1
m.next(10) = (1 + 10) / 2
m.next(3) = (1 + 10 + 3) / 3
m.next(5) = (10 + 3 + 5) / 3

class MovingAverage {
    Queue<Integer> q;
    double sum = 0;
    int size;

    public MovingAverage(int s) {
        q = new LinkedList();
        size = s;
    }
    
    public double next(int val) {
        if(q.size() == size){
            sum = sum - q.poll();
        }
        q.offer(val);
        sum += val;
        return sum/q.size();
    }
}

时间复杂度 O(1)

空间复杂度 O(N)