Binary Tree Zigzag Level Order Traversal

https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/description/

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).

Thoughts

逐层正着倒着加入.

Code

/**
 * 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 ArrayList<>();
        if (root == null) {
            return res;
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        boolean clock = true;
        while (!queue.isEmpty()) {
            int length = queue.size();
            List<Integer> subres = new ArrayList<>();
            for (int i = 0; i < length; i++) {
                TreeNode node = queue.poll();
                if (clock) {
                    subres.add(node.val);
                } else {
                    subres.add(0, node.val);
                }
                if (node.left != null) queue.add(node.left);
                if (node.right != null) queue.add(node.right);
            }
            clock = clock ? false : true;
            res.add(subres);
        }
        return res;
    }
}

Analysis

时空复杂度O(N).