103. Binary Tree Zigzag Level Order Traversal

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new LinkedList<>();
        if (root == null) {
            return res;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int layer = 1;
        while (!queue.isEmpty()) {
            List<Integer> cur = new LinkedList<>();
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode curNode = queue.poll();
                if (layer % 2 == 1) {
                    cur.add(curNode.val);
                } else {
                    cur.add(0, curNode.val);
                }
                if (curNode.left != null) {
                    queue.offer(curNode.left);
                }
                if (curNode.right != null) {
                    queue.offer(curNode.right);
                }         

            }
            res.add(cur);
            layer ++;
        }
        return res;
    }
}