堆 - Java

class creatH {
    public int[] elem;
    public int usedSize;

    public creatH(){
        this.elem = new int[10];
    }

    //根节点从0开始编号的情况下
    //parent每棵树的根节点，len每棵树的调整的结束位置
    public void shiftDown(int parent,int len){
        int child = 2*parent+1;
        while(child < len){//至少有一个孩子
            if(child+1 < len && elem[child] < elem[child+1]){
                child++;
            }else{
                if(elem[child] > elem[parent]){
                    int tmp = elem[parent];
                    elem[parent] = elem[child];
                    elem[child] = tmp;
                    parent = child;
                    child = parent*2+1;
                }else{
                    break;
                }
            }
        }
    }

    public void creatHeap(int[] arr){
        for (int i = 0; i < arr.length; i++) {
            elem[i] = arr[i];
            usedSize++;
        }
        for (int parent = (usedSize-1-1)/2; parent >= 0; parent--) {
            //调整
            shiftDown(parent,usedSize);
        }
    }
}

    public void shiftUp(int child){
        int parent = (child-1)/2;
        while(child > 0){
            if(elem[child] > elem[parent]){
                int tmp = elem[child];
                elem[child] = elem[parent];
                elem[parent] = tmp;
                parent = (child-1)/2;
            }else{
                break;
            }
        }
    }
    public void offer(int child,int val){
        if(isFull()){
            //扩容
            elem = Arrays.copyOf(elem,2*elem.length);
        }
        elem[usedSize++] = val;
        //传入的是usedSize-1
        shiftUp(usedSize-1);
    }
    public boolean isFull(){
        return usedSize == elem.length;
    }

    public boolean isEmpty(){
        return usedSize == 0;
    }
    
    public int poll(){
        if(isEmpty()){
            throw new RuntimeException("优先级队列为空!");
        }
        int tmp = elem[0];//每次都出最大或最小的
        elem[0] = elem[usedSize-1];
        elem[usedSize-1] = tmp;
        usedSize--;
        shiftDown(0,usedSize);
        return tmp;
    }

    //返回队首元素
    public int peek(){
        if(isEmpty()){
            throw new RuntimeException("优先级队列为空!");
        }
        return elem[0];//0下标就是队首元素
    }