自定义优先级队列PriorityQueue

搞得简单一点。主要是heapInsert（往堆里面插入数据），heapify（调整堆）这两个函数。

/**
 * @Author lyuWalle
 * @Date 2020/9/16 11:05
 * @Version 1.0
 * 简单自定义优先级队列，采用ArrayList实现。
 * 主要实现：
 * add(Integer i)
 * peek();
 * poll();
 * size();
 * 
 */
public class MyPriorityQueue {
    private int cap;
    private List<Integer> list;
    public MyPriorityQueue(){
        this.cap = 10;
        list = new ArrayList<>(10);
    }
    public MyPriorityQueue(int capacity){
        this.cap = capacity;
        list = new ArrayList<>(this.cap);
    }
    public void add(int num){   //将一个新的值插入到大根堆里面
        list.add(num);
        heapInsert(list, list.size() - 1);
    }

    private void heapInsert(List<Integer> list, int i) {
        while(list.get(i) > list.get((i - 1)/2)){
            Collections.swap(list, i, (i - 1)/2);
            i = (i - 1)/2;
        }
    }

    public void heapify(List<Integer> list, int top, int end){
        int left = top * 2 + 1;
        while(left < end){
            int largest = left + 1 <= end && list.get(left + 1) > list.get(left) ? left + 1 : left;
            if(list.get(largest) > list.get(top)){
                Collections.swap(list, top, largest);
            }else{
                break;  //说明不需要调整堆
            }
            top = largest;
            left = 2 * top + 1;
        }
    }

    public int peek(){
        if(list.size()>0){
            return list.get(0);
        }else{
            return -1;  //假设队列为空时返回-1
        }

    }
    public int poll(){
        if(list.size()>0){
            int top = list.get(0);
            list.remove(0);
            heapify(list, 0, list.size() - 1);
            return top;
        }else{
            return -1;
        }
    }
    public int size(){
        return list.size();
    }

    //for test
    public static void main(String[] args) {
        MyPriorityQueue mpq = new MyPriorityQueue();
        mpq.add(1);
        mpq.add(2);
        mpq.add(3);
        mpq.add(4);
        System.out.println(mpq.peek());
        mpq.poll();
        System.out.println(mpq.peek());
        System.out.println(mpq.size());
        mpq.add(5);
        System.out.println(mpq.poll());
        System.out.println(mpq.peek());
    }
}