Tree ZigZag Traversal, Java

Tree ZigZag Traversal

//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.*;
import java.lang.*;

//TREE ZIGZAG TRAVERSAL

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}

class Rextester
{  
    public static void main(String args[])
    {
        System.out.println("Hello, World!");
        TreeNode root = new TreeNode(3);
        root.left = new TreeNode(9);
        root.right = new TreeNode(20);
        root.right.left = new TreeNode(15);
        root.right.right = new TreeNode(7);
        zigzagLevelOrder(root);

}

public static List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> out = new ArrayList<List<Integer>>();        
        helper(root,out,0);
        return out;                
        
    }

public static void helper(TreeNode root,List<List<Integer>> out,int level){
        if(root==null)
            return;

if(out.size()<=level)
            out.add(new ArrayList<>());

List<Integer> lst = out.get(0);
        if(level%2==0)
            lst.add(root.val);
        else
            lst.add(0,root.val);
        
        helper(root.left,out,level+1);
        helper(root.right,out,level+1);

}
}


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