我尝试使用 java 构建一个 minHeap,这是我的代码:
public class MyMinHeap {
private ArrayList<Node> heap;
public MyMinHeap() {
heap = new ArrayList<Node>();
}
public MyMinHeap(ArrayList<Node> nodeList) {
heap = nodeList;
buildHeap();
}
public void buildHeap() {
int i = heap.size() / 2;
while (i >= 0) {
minHeapify(i);
i--;
}
}
public Node extractMin() {
if (heap.size() <= 0) return null;
Node minValue = heap.get(0);
heap.set(0, heap.get(heap.size() - 1));
heap.remove(heap.size() - 1);
minHeapify(0);
return minValue;
}
public String toString() {
String s = "";
for (Node n : heap) {
s += n + ",";
}
return s;
}
public void minHeapify(int i) {
int left = 2 * i + 1;
int right = 2 * i + 2;
int smallest = i;
if (left < heap.size() - 1 && lessThan(left, smallest))
smallest = left;
if (right < heap.size() - 1 && lessThan(right, smallest))
smallest = right;
if (smallest != i) {
swap(smallest, i);
minHeapify(smallest);
}
}
private void swap(int i, int j) {
Node t = heap.get(i);
heap.set(i, heap.get(j));
heap.set(j, t);
}
public boolean lessThan(int i, int j) {
return heap.get(i)
.compareTo(heap.get(j)) < 0;
}
public static void main(String[] args) {
char[] chars = {'a', 'b', 'c', 'd', 'e', 'f'};
int[] freqs = {45, 13, 12, 16, 9, 5};
ArrayList<Node> data = new ArrayList<Node>();
for (int i = 0; i < chars.length; i++) {
data.add(new Node(chars[i], freqs[i]));
}
MyMinHeap heap = new MyMinHeap(data);
System.out.println("print the heap : " + heap);
for (int i = 0; i < chars.length; i++) {
System.out.println("Smallest is :" + heap.extractMin());
}
}
}
输出应该是:5,9,12,13,16,45,
但我得到的是:9,13,12,16,45
我已经调试过了,但还是搞不清楚,有人帮忙吗?非常感谢。
最佳答案
插入: 当我们插入最小堆时,我们总是从底部插入元素开始。我们插入 最右边的位置,以保持完整的树属性。 然后,我们通过将新元素与其父元素交换来“修复”树,直到找到合适的位置 元素。我们基本上冒泡了最小元素。 这需要 0 (log n) 时间,其中 n 是堆中的节点数。
提取最小元素: 找到最小堆的最小元素很容易:它总是在顶部。棘手的部分是如何删除 它。 (事实上 ,这并不那么棘手。) 首先,我们删除最小元素并将其与堆中的最后一个元素(最底部, 最右边的元素)。然后,我们冒泡这个元素,将它与它的一个 child 交换,直到 minheap 属性(property)恢复。 我们是用左 child 还是右 child 交换它?这取决于他们的值(value)观。没有固有的 在左右元素之间排序,但您需要取较小的元素以保持 最小堆排序。
public class MinHeap {
private int[] heap;
private int size;
private static final int FRONT = 1;
public MinHeap(int maxSize) {
heap = new int[maxSize + 1];
size = 0;
}
private int getParent(int position) {
return position / 2;
}
private int getLeftChild(int position) {
return position * 2;
}
private int getRightChild(int position) {
return position * 2 + 1;
}
private void swap(int position1, int position2) {
int temp = heap[position1];
heap[position1] = heap[position2];
heap[position2] = temp;
}
private boolean isLeaf(int position) {
if (position > size / 2) {
return true;
}
return false;
}
public void insert(int data) {
heap[++size] = data;
int currentItemIndex = size;
while (heap[currentItemIndex] < heap[getParent(currentItemIndex)]) {
swap(currentItemIndex, getParent(currentItemIndex));
currentItemIndex = getParent(currentItemIndex);
}
}
public int delete() {
int item = heap[FRONT];
swap(FRONT, size--); // heap[FRONT] = heap[size--];
heapify(FRONT);
return item;
}
private void heapify(int position) {
if (isLeaf(position)) {
return;
}
if (heap[position] > heap[getLeftChild(position)]
|| heap[position] > heap[getRightChild(position)]) {
// if left is smaller than right
if (heap[getLeftChild(position)] < heap[getRightChild(position)]) {
// swap with left
swap(heap[position], heap[getLeftChild(position)]);
heapify(getLeftChild(position));
} else {
// swap with right
swap(heap[position], heap[getRightChild(position)]);
heapify(getRightChild(position));
}
}
}
@Override
public String toString() {
StringBuilder output = new StringBuilder();
for (int i = 1; i <= size / 2; i++) {
output.append("Parent :" + heap[i]);
output
.append("LeftChild : " + heap[getLeftChild(i)] + " RightChild :" + heap[getRightChild(i)])
.append("\n");
}
return output.toString();
}
public static void main(String... arg) {
System.out.println("The Min Heap is ");
MinHeap minHeap = new MinHeap(15);
minHeap.insert(5);
minHeap.insert(3);
minHeap.insert(17);
minHeap.insert(10);
minHeap.insert(84);
minHeap.insert(19);
minHeap.insert(6);
minHeap.insert(22);
minHeap.insert(9);
System.out.println(minHeap.toString());
System.out.println("The Min val is " + minHeap.delete());
}
}
关于java - 使用java构建最小堆,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/27586437/