programming-examples/java/Data_Structures/TreapBST.java

import java.util.*;

// https://en.wikipedia.org/wiki/Treap
public class TreapBST {
	static Random random = new Random();

	static class Treap {
		int key;
		long prio;
		Treap left;
		Treap right;
		int size;

		Treap(int key) {
			this.key = key;
			prio = random.nextLong();
			size = 1;
		}

		void update() {
			size = 1 + getSize(left) + getSize(right);
		}
	}

	static int getSize(Treap root) {
		return root == null ? 0 : root.size;
	}

	static class TreapPair {
		Treap left;
		Treap right;

		TreapPair(Treap left, Treap right) {
			this.left = left;
			this.right = right;
		}
	}

	static TreapPair split(Treap root, int minRight) {
		if (root == null)
			return new TreapPair(null, null);
		if (root.key >= minRight) {
			TreapPair leftSplit = split(root.left, minRight);
			root.left = leftSplit.right;
			root.update();
			leftSplit.right = root;
			return leftSplit;
		} else {
			TreapPair rightSplit = split(root.right, minRight);
			root.right = rightSplit.left;
			root.update();
			rightSplit.left = root;
			return rightSplit;
		}
	}

	static Treap merge(Treap left, Treap right) {
		if (left == null)
			return right;
		if (right == null)
			return left;
		// if (random.nextInt(left.size + right.size) < left.size) {
		if (left.prio > right.prio) {
			left.right = merge(left.right, right);
			left.update();
			return left;
		} else {
			right.left = merge(left, right.left);
			right.update();
			return right;
		}
	}

	static Treap insert(Treap root, int key) {
		TreapPair t = split(root, key);
		return merge(merge(t.left, new Treap(key)), t.right);
	}

	static Treap insert2(Treap root, int key) { // alternative implementation
		return insert_(root, new Treap(key));
	}

	static Treap insert_(Treap root, Treap node) {
		if (root == null) {
			return node;
		}
		if (root.prio < node.prio) {
			TreapPair t = split(root, node.key);
			node.left = t.left;
			node.right = t.right;
			node.update();
			return node;
		}
		if (node.key < root.key) {
			root.left = insert_(root.left, node);
			root.update();
			return root;
		} else {
			root.right = insert_(root.right, node);
			root.update();
			return root;
		}
	}

	static Treap remove(Treap root, int key) {
		TreapPair t = split(root, key);
		return merge(t.left, split(t.right, key + 1).right);
	}

	static Treap remove2(Treap root, int key) { // alternative implementation
		if (root == null) {
			return null;
		}
		if (key < root.key) {
			root.left = remove(root.left, key);
			root.update();
			return root;
		} else if (key > root.key) {
			root.right = remove(root.right, key);
			root.update();
			return root;
		} else {
			return merge(root.left, root.right);
		}
	}

	static int kth(Treap root, int k) {
		if (k < getSize(root.left))
			return kth(root.left, k);
		else if (k > getSize(root.left))
			return kth(root.right, k - getSize(root.left) - 1);
		return root.key;
	}

	static void print(Treap root) {
		if (root == null)
			return;
		print(root.left);
		System.out.println(root.key);
		print(root.right);
	}

	// O(n) treap creation (if not counting keys sorting)
	// http://wcipeg.com/wiki/Cartesian_tree
	static Treap createTreap(int[] keys) {
		Treap[] nodes = Arrays.stream(keys).mapToObj(Treap::new).sorted((a, b) -> Integer.compare(a.key, b.key)).toArray(Treap[]::new);
		int n = keys.length;
		int[] parent = new int[n];
		Arrays.fill(parent, -1);
		Treap root = n > 0 ? nodes[0] : null;
		for (int i = 1; i < n; i++) {
			int last = i - 1;
			if (nodes[last].prio >= nodes[i].prio) {
				nodes[last].right = nodes[i];
				parent[i] = last;
			} else {
				while (nodes[last].prio < nodes[i].prio && parent[last] != -1) {
					last = parent[last];
				}
				if (nodes[last].prio < nodes[i].prio) {
					nodes[i].left = nodes[last];
					root = nodes[i];
				} else {
					nodes[i].left = nodes[last].right;
					nodes[last].right = nodes[i];
					parent[i] = last;
				}
			}
		}
		updateSizes(root);
		return root;
	}

	private static void updateSizes(Treap root) {
		if (root == null)
			return;
		updateSizes(root.left);
		updateSizes(root.right);
		root.update();
	}

	// random test
	public static void main(String[] args) {
		long time = System.currentTimeMillis();
		Treap treap = null;
		Set<Integer> set = new TreeSet<>();
		for (int i = 0; i < 1000_000; i++) {
			int key = random.nextInt(100_000);
			if (random.nextBoolean()) {
				treap = remove(treap, key);
				set.remove(key);
			} else if (!set.contains(key)) {
				treap = insert(treap, key);
				set.add(key);
			}
			if (set.size() != getSize(treap))
				throw new RuntimeException();
		}
		for (int i = 0; i < getSize(treap); i++) {
			if (!set.contains(kth(treap, i)))
				throw new RuntimeException();
		}
		System.out.println(System.currentTimeMillis() - time);

		for (int step = 0; step < 1000; step++) {
			int n = random.nextInt(10) + 1;
			int[] keys = random.ints(n, 0, 100).toArray();
			Treap t = createTreap(keys);
			Arrays.sort(keys);
			for (int i = 0; i < n; i++) {
				if (kth(t, i) != keys[i])
					throw new RuntimeException();
			}
			checkHeapInvariant(t);
		}
	}

	private static void checkHeapInvariant(Treap root) {
		if (root == null)
			return;
		if (root.left != null && root.left.prio > root.prio)
			throw new RuntimeException();
		if (root.right != null && root.right.prio > root.prio)
			throw new RuntimeException();
		checkHeapInvariant(root.left);
		checkHeapInvariant(root.right);
	}
}