103. Binary Tree Zigzag Level Order Traversal

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).

For example:
```
Given binary tree [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
return its zigzag level order traversal as:
[
  [3],
  [20,9],
  [15,7]
]
```
```java

/**
 * Definition for a binary tree node.
 * public 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>> res=new ArrayList<>();
        if(root==null) return res;
        Queue<TreeNode> queue=new LinkedList<>();
        queue.offer(root);
        boolean dir=true;
        while(!queue.isEmpty()){
            int size=queue.size();
            List<Integer> list=new ArrayList<>();
            for(int i=0;i<size;i++){
                TreeNode curr=queue.poll();
                if(dir){
                    list.add(curr.val);
                    if(curr.left!=null) queue.offer(curr.left);
                    if(curr.right!=null) queue.offer(curr.right);
                } else{
                    list.add(0,curr.val);
                    if(curr.left!=null) queue.offer(curr.left);
                    if(curr.right!=null) queue.offer(curr.right);
                }
            }
            res.add(list);
            dir=!dir;
        }
        return res;
    }
}
```