ZigZag_Tree_Traversal.java

import java.util.*;
import java.util.HashMap;
import java.io.*;

class Node {
  int data;
  Node left, right;

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

public class GFG2 {
  static Node buildTree(String str) {

    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;
  }

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

  /* Drier program to test above functions */
  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);
      GFG g = new GFG();

      ArrayList<Integer> res = g.zigZagTraversal(root);
      for (int i = 0; i < res.size(); i++)
        System.out.print(res.get(i) + " ");
      System.out.println();

      t--;

    }
  }
}

class GFG {
  ArrayList<Integer> zigZagTraversal(Node root) {
    ArrayList<Integer> ans = new ArrayList<>();
    if (root == null)
      return ans;
    Stack<Node> st1 = new Stack<>();
    Stack<Node> st2 = new Stack<>();
    Boolean leftToRight = true;

    st1.push(root);
    while (!st1.isEmpty()) {
      Node curr = st1.pop();
      ans.add(curr.data);

      if (leftToRight) {
        if (curr.left != null)
          st2.push(curr.left);
        if (curr.right != null)
          st2.push(curr.right);
      } else {
        if (curr.right != null)
          st2.push(curr.right);
        if (curr.left != null)
          st2.push(curr.left);
      }

      if (st1.isEmpty()) {
        leftToRight = !leftToRight;
        Stack<Node> temp = new Stack<>();
        temp = st1;
        st1 = st2;
        st2 = temp;
      }
    }
    return ans;
  }
}