基于动态数组实现最大堆

package structures;

public class MaxHeap<E extends Comparable<E>> {
    private Array<E> data;

    public MaxHeap(int capacity) {
        data = new Array<>(capacity);
    }

    public MaxHeap() {
        this(10);
    }

    public MaxHeap(E[] arr) {
        data = new Array<>(arr);
        for (int i = parent(arr.length - 1); i >= 0; i--) {
            siftDown(i);
        }
    }

    public boolean isEmpty() {
        return data.isEmpty();
    }

    public int getSize() {
        return data.getSize();
    }

    private int parent(int index) {
        if (index <= 0) {
            throw new IllegalArgumentException("index is not illegal");
        }

        return (index - 1) / 2;
    }

    private int leftChild(int index) {
        return index * 2 + 1;
    }

    private int rightChild(int index) {
        return index * 2 + 2;
    }

    public void add(E e) {
        data.addLast(e);
        siftUp(data.getSize() - 1);
    }

    private void siftUp(int k) {
        while (k > 0 && data.get(parent(k)).compareTo(data.get(k)) < 0) {
            data.swap(k, parent(k));
            k = parent(k);
        }
    }

    public E extractMax() {
        E ret = findMax();
        data.swap(0, data.getSize() - 1);
        data.removeLast();
        siftDown(0);
        return ret;
    }

    private void siftDown(int k) {
        while (leftChild(k) < data.getSize()) {
            int j = leftChild(k);
            //找到k孩子节点中最大值的节点 j+1代表右孩子
            if (j + 1 < data.getSize() && data.get(j + 1).compareTo(data.get(j)) > 0) {
                //说明有右孩子 且右孩子值最大
                j ++; //代表j是孩子节点最大值的索引
            }

            if (data.get(k).compareTo(data.get(j)) >= 0) {
                //k索引上的值最大 不需交换 退出循环
                break;
            }

            data.swap(k, j);
            k = j;
        }
    }

    public E findMax() {
        if (data.isEmpty()) {
            throw new IllegalArgumentException("heap is empty");
        }

        return data.get(0);
    }

    //替换掉最大值
    public E replace(E e) {
        E ret = findMax();
        data.set(0, e);
        siftDown(0);
        return ret;
    }
}