/**
Definition for a binary tree node.*/
import java.util.*;
 
   class TreeNode {
 
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode() {}
      TreeNode(int val) { this.val = val; }
      TreeNode(int val, TreeNode left, TreeNode right) {
          this.val = val;
          this.left = left;
          this.right = right;
      }
  }
class Solution {
 
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
      List<List<Integer>> output = new ArrayList<>();
 
      if(root == null)
        return output;
 
      Stack<TreeNode> parent = new Stack<>();
      Stack<TreeNode> child = new Stack<>();
      boolean leftToRight = true;
 
      parent.add(root);
 
      List<Integer> level = new ArrayList<>();
      while(!parent.isEmpty()){
        TreeNode node = parent.pop();
        level.add(node.val);
        if(leftToRight){
                if(node.left!=null) child.add(node.left);
                if(node.right!=null) child.add(node.right);
        }
        else{
                if(node.right!=null) child.add(node.right);
                if(node.left!=null) child.add(node.left);
        }
        if(parent.isEmpty()){
          leftToRight = !leftToRight;
          parent = child;
          child = new Stack<>();
          output.add(level);
          level = new ArrayList<>();
        }
      }
      return output;
    }
 
    public static void main(String args[]){
    	TreeNode root = new TreeNode(1);
    	root.left = new TreeNode(2);
    	root.right = new TreeNode(3);
    	root.left.left = new TreeNode(4);
    	root.left.right = new TreeNode(5);
    	root.right.left = new TreeNode(6);
    	root.right.right = new TreeNode(7);
 
    	System.out.print(new Solution().zigzagLevelOrder(root));
 
    }
 
}