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1 借助堆实现优先队列
Queue.java
package heap;
public interface Queue<E> {
int getSize();
boolean isEmpty();
void enqueue(E e);
E dequeue();
E getFront();
}
MaxHeap.java
package heap;
public class MaxHeap<E extends Comparable<E>> {
private Array<E> data;
public MaxHeap(int capacity) {
data = new Array<E>(capacity);
}
public MaxHeap(E[] arr) {
data = new Array<>(arr);
for (int i = parent(arr.length - 1); i >= 0; i--) {
siftDown(i);
}
}
public MaxHeap() {
data = new Array<>();
}
public int size() {
return data.getSize();
}
public boolean isEmpty() {
return data.isEmpty();
}
// 返回完全二叉树的数组表示中,一个索引所表示的元素的父亲节点的索引
private int parent(int index) {
if (index == 0) {
throw new IllegalArgumentException("index-0 does't have parent");
}
return (index - 1) / 2;
}
// 返回完全二叉树的数组表示中,一个索引所表示的元素的左孩子节点的索引
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);
}
}
// 查看堆中最大元素
public E findMax() {
if (data.getSize() == 0) {
throw new IllegalArgumentException("heap is empty");
}
return data.get(0);
}
// 取出堆中最大的元素
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);
if (j + 1 < data.getSize() &&
data.get(j + 1).compareTo(data.get(j)) > 0) {
j = rightChild(k);
// data[j] 是 leftChild 和 rightChild 中的最大值
}
if (data.get(k).compareTo(data.get(j)) >= 0) {
break;
}
data.swap(k, j);
k = j;
}
}
/*
* 取出堆中最大元素,并且替换成元素 e
*
* */
public E replace(E e) {
E ret = findMax();
data.set(0, e);
siftDown(0);
return ret;
}
}
PriorityQueue.java
package heap;
public class PriorityQueue<E extends Comparable<E>> implements Queue<E> {
private MaxHeap<E> maxHeap;
public PriorityQueue(){
maxHeap = new MaxHeap<>();
}
@Override
public int getSize() {
return maxHeap.size();
}
@Override
public boolean isEmpty() {
return maxHeap.isEmpty();
}
@Override
public void enqueue(E e) {
maxHeap.add(e);
}
@Override
public E dequeue() {
return null;
}
@Override
public E getFront() {
return maxHeap.findMax();
}
}