题目

给你二叉树的根节点 root ，返回其节点值的 锯齿形层序遍历 。（即先从左往右，再从右往左进行下一层遍历，以此类推，层与层之间交替进行）。

示例 1：

输入：root = [3,9,20,null,null,15,7]
输出：[[3],[20,9],[15,7]]

解

class Solution {public List<List<Integer>> zigzagLevelOrder(TreeNode root) {List<List<Integer>> result = new ArrayList<>();if (root == null) {return result;}LinkedList<TreeNode> queue = new LinkedList<>();LinkedList<TreeNode> queue2 = new LinkedList<>();queue.offer(root);boolean flag = true;LinkedList<Integer> res = new LinkedList<>();while (!queue.isEmpty()) {TreeNode poll = queue.poll();if (flag) {res.add(poll.val);} else {res.addFirst(poll.val);}if (poll.left != null) {queue2.offer(poll.left);}if (poll.right != null) {queue2.offer(poll.right);}if (queue.isEmpty()) {flag = !flag;result.add(res);res = new LinkedList<>();queue = queue2;queue2 = new LinkedList<>();}}return result;}
}