Binary_Tree_to_DLL.java

// { Driver Code Starts
//Initial Template for Java

import java.util.Scanner;
import java.util.*;
import java.io.*;
import java.lang.*;

class Node {
  Node left, right;
  int data;

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

}

class BT_To_DLL {

  static Node buildTree(String str) throws IOException {

    if (str.length() == 0 || str.charAt(0) == 'N') {
      return null;
    }

    String ip[] = str.split(" ");
    // Create the root of the tree
    Node root = new Node(Integer.parseInt(ip[0]));
    // Push the root to the queue

    Queue<Node> queue = new LinkedList<>();

    queue.add(root);
    // Starting from the second element

    int i = 1;
    while (queue.size() > 0 && i < ip.length) {

      // Get and remove the front of the queue
      Node currNode = queue.peek();
      queue.remove();

      // Get the current node's value from the string
      String currVal = ip[i];

      // If the left child is not null
      if (!currVal.equals("N")) {

        // Create the left child for the current node
        currNode.left = new Node(Integer.parseInt(currVal));
        // Push it to the queue
        queue.add(currNode.left);
      }

      // For the right child
      i++;
      if (i >= ip.length)
        break;

      currVal = ip[i];

      // If the right child is not null
      if (!currVal.equals("N")) {

        // Create the right child for the current node
        currNode.right = new Node(Integer.parseInt(currVal));

        // Push it to the queue
        queue.add(currNode.right);
      }
      i++;
    }

    return root;
  }

  void inorder(Node node) {
    if (node == null)
      return;
    else
      inorder(node.left);
    System.out.print(node.data + " ");
    inorder(node.right);
  }

  public static void printList(Node head) {
    Node prev = head;
    while (head != null) {
      System.out.print(head.data + " ");
      prev = head;
      head = head.right;
    }

    System.out.println();
    while (prev != null) {
      System.out.print(prev.data + " ");
      prev = prev.left;
    }
  }

  public static void main(String args[]) throws IOException {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

    int t = Integer.parseInt(br.readLine());

    while (t > 0) {
      String s = br.readLine();
      Node root = buildTree(s);
      Solution g = new Solution();

      Node ans = g.bToDLL(root);
      printList(ans);
      t--;
      System.out.println();
    }

  }
}


class Solution {
  public Node head = null;
  public Node prev = null;

  Node bToDLL(Node root) {
    if (root == null)
      return null;

    bToDLL(root.left);

    if (head == null) {
      head = root;
      prev = root;
    } else {
      root.left = prev;
      prev.right = root;
      prev = root;
    }

    bToDLL(root.right);

    return head;
  }
}