/*
 * @lc app=leetcode id=103 lang=java
 *
 * [103] Binary Tree Zigzag Level Order Traversal
 */

// @lc code=start
/**
 * 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) {
        if(root == null) {
            return new ArrayList<>();
        }
        LinkedList<TreeNode> queue = new LinkedList<>();
        ArrayList<TreeNode> stack = new ArrayList<>();
        queue.add(root);
        List<List<Integer>> ret = new ArrayList<>();
        List<Integer> curLayer = new ArrayList<>();
        TreeNode lastLayerNode = null, curLayerNode;
        while(!queue.isEmpty()) {
            TreeNode tmp = queue.pollFirst();
            curLayer.add(tmp.val);
            if(tmp.left != null) {
                // queue.add(tmp.left);
                stack.addLast(tmp.left);
            }
            if(tmp.right != null) {
                // queue.add(tmp.right);
                stack.addLast(tmp.right);
            }
            if(tmp == lastLayerNode || lastLayerNode == null) {
                // while()
                // ret.add(curLayer);
                // curLayer = new ArrayList<>();
                // lastLayerNode = curLayerNode;
                while(!stack.isEmpty()) {
                    curLayerNode = stack.pollLast();
                    queue.add(curLayerNode);
                }
                lastLayerNode = curLayerNode;
                ret.add(curLayer);
                curLayer = new ArrayList<>();
            }
        }
        return ret;
    }
}
// @lc code=end