优先队列的二叉堆Java实现

package practice;


import edu.princeton.cs.algs4.StdRandom;

public class TestMain {
    public static void main(String[] args) {
        int[] a = new int[20];
        for (int i = 0; i < a.length; i++) {
            int temp = (int)(StdRandom.random()*100);
            a[i] = temp;
        }
        
        for (int i : a) {
            System.out.print(i+" ");
        }
        System.out.println();
        
        PQHeap pq = new PQHeap();
        for (int i = 0; i < 20; i++) {
            pq.insert(a[i]);
        }
        System.out.println();
        for (int i = 0; i < 20; i++) {
            System.out.print(pq.delMax()+" ");
        }
        
    }
}
/*
 * 优先队列的堆实现
 * 二叉堆，每个元素有两个子元素，两个子元素均比自己小
 */
class PQHeap{
    private int[] a;
    private int p = 1;
    public PQHeap() {
        a = new int[2];
    }
    /*
     * 插入元素
     */
    public void insert(int elements) {
        if (p == a.length) { resize(a.length*2); }
        a[p++] = elements;
        swim(p - 1); //将刚插入的元素上浮到正确位置
    }
    /*
     * 返回并删除最大元素
     */
    public int delMax() {
        if (p == a.length/4) { resize(a.length/2); }
        int result = a[1]; //找出最大元素
        exch(1, --p); //将最后一个元素移到顶端
        a[p] = 0; 
        sink(1); //将刚移上去的元素沉下去，使堆有序
        return result;
    }
    
    public boolean isEmpty() {
        return p == 0;
    }
    
    public int size() {
        return p;
    }
    
    public void show() {
        for (int i : a) {
            System.out.print(i+" ");
        }
        System.out.println();
    }
    /*
     * 上浮元素
     */
    private void swim(int k) { //将元素与父元素比较，比父元素大则换位置
        while (k > 1 && a[k/2] < a[k]) { 
            exch(k/2, k);
            k = k/2;
        }
    }
    
    private void sink(int k) { //将元素与子元素比较，比子元素小则和两个中较大的那个换位置
        while (2*k < p && (a[k] < a[2*k + 1]) || (a[k] < a[2*k])) {
            if (a[2*k + 1] > a[2*k]) { exch(k, 2*k + 1); k = 2*k + 1; }
            else                     { exch(k, 2*k); k = 2*k; }
        }
    }
    
    private void resize(int length) {
        int[] b = new int[length]; //将数组长度改变
        for (int i = 0; i < p; i++) { //将数组复制
            b[i] = a[i];
        }
        a = b; //让a指向b的内存空间
    }
    
    /*
     * 交换
     */
    private void exch (int i, int j) {
        int t = a[i];
        a[i] = a[j];
        a[j] = t;
    }
}