tree.java

import java.util.LinkedList;
import java.util.Queue;

public class tree {
    static class Node {
        int data;
        Node left;
        Node right;

        Node(int data) {
            this.data = data;
            this.left = null;
            this.right = null;
        }
    }

    static class Binarytree {
        static int idx = -1;

        public static Node tree(int node[]) {
            idx++;
            if (node[idx] == -1) {
                return null;
            }
            Node newNode = new Node(node[idx]);
            newNode.left = tree(node);
            newNode.right = tree(node);
            return newNode;
        }
    }

    public static void preorder(Node root) {
        if (root == null) {
            return;
        }
        System.out.print(root.data);
        preorder(root.left);
        preorder(root.right);
    }

    public static void inorder(Node root) {
        if (root == null) {
            return;
        }
        inorder(root.left);
        System.out.print(root.data);
        inorder(root.right);
    }

    public static void postorder(Node root) {
        if (root == null) {
            return;
        }
        postorder(root.left);
        postorder(root.right);
        System.out.print(root.data);
    }

    public static int countNode(Node root) {
        if (root == null) {
            return 0;
        }
        return countNode(root.left) + countNode(root.right) + 1;
    }

    public static void level_order(Node root) {
        if (root == null) {
            return;
        }
        Queue<Node> q = new LinkedList<>();
        q.add(root);
        
        while (!q.isEmpty()) {
            Node curr = q.remove();
            if (curr == null) {
                System.out.println(curr);
                if (q.isEmpty()) {
                    break;
                } else {
                    q.add(null);
                }
            } else {
                System.out.print(curr.data);
                if (curr.left != null) {
                    q.add(curr.left);
                }
                if (curr.right != null) {
                    q.add(curr.right);
                }
            }
        }
    }

    public static void main(String[] args) {
        int node[] = { 1, 2, 4, -1, -1, -1, -1 };
        Node root = Binarytree.tree(node);
        // preorder(root);
        // System.out.println();
        // inorder(root);
        // System.out.println();
        // postorder(root);
        // System.err.println();
        // System.out.println(countNode(root));
        level_order(root);

    }
}