# Binary Tree Zigzag Level Order Traversal

## Problem

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

## Example

Given binary tree {3,9,20,#,#,15,7},

        3
/ \
9  20
/  \
15   7

return its zigzag level order traversal as:

[
[3],
[20,9],
[15,7]
]

## Code - Java

/**
* Definition of TreeNode:
* public class TreeNode {
*   public int val;
*   public TreeNode left, right;
*   public TreeNode(int val) {
*     this.val = val;
*     this.left = this.right = null;
*   }
* }
*/
public class Solution {
/**
* @param root: The root of binary tree.
* @return: A list of lists of integer include
* the zigzag level order traversal of its nodes' values
*/
public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
ArrayList<ArrayList<Integer>> result = new ArrayList<>();
traverse(result, root, 0);
for (int i = 1; i < result.size(); i += 2) {
int len = result.get(i).size();
List<Integer> list = result.get(i);
for (int j = 0; j < len / 2; j++) {
int tmp = list.get(j);
list.set(j, list.get(len - 1 - j));
list.set(len - 1 - j, tmp);
}
}
return result;
}

private void traverse(ArrayList<ArrayList<Integer>> result, TreeNode node, int level) {
if (node == null) {
return;
}
while (result.size() < level + 1) {
}