83 lines
1.6 KiB
Java
83 lines
1.6 KiB
Java
import java.util.*;
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// https://en.wikipedia.org/wiki/Binary_heap
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// invariant: heap[parent] <= heap[child]
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public class BinaryHeap {
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int[] heap;
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int size;
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public BinaryHeap(int n) {
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heap = new int[n];
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}
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// build heap in O(n)
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public BinaryHeap(int[] values) {
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heap = values.clone();
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size = values.length;
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for (int pos = size / 2 - 1; pos >= 0; pos--)
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down(pos);
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}
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public int removeMin() {
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int removed = heap[0];
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heap[0] = heap[--size];
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down(0);
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return removed;
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}
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public void add(int value) {
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heap[size] = value;
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up(size++);
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}
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void up(int pos) {
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while (pos > 0) {
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int parent = (pos - 1) / 2;
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if (heap[pos] >= heap[parent])
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break;
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swap(pos, parent);
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pos = parent;
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}
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}
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void down(int pos) {
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while (true) {
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int child = 2 * pos + 1;
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if (child >= size)
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break;
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if (child + 1 < size && heap[child + 1] < heap[child])
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++child;
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if (heap[pos] <= heap[child])
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break;
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swap(pos, child);
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pos = child;
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}
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}
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void swap(int i, int j) {
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int t = heap[i];
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heap[i] = heap[j];
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heap[j] = t;
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}
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// random test
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public static void main(String[] args) {
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Random rnd = new Random(1);
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for (int step = 0; step < 1000; step++) {
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int n = rnd.nextInt(100) + 1;
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PriorityQueue<Integer> q = new PriorityQueue<>();
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BinaryHeap h = new BinaryHeap(n);
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for (int op = 0; op < 1000; op++) {
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if (rnd.nextBoolean() && q.size() < n) {
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int v = rnd.nextInt();
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q.add(v);
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h.add(v);
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} else if (!q.isEmpty()) {
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if (q.remove() != h.removeMin())
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throw new RuntimeException();
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}
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}
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}
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}
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}
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