Binary Tree Zigzag Level Order Traversal leetcode

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,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

[
  [3],
  [20,9],
  [15,7]
]

用一个flag记录是否要reverse 一层reverse 一层不reverse添加
判断flag要在for 循环外面
时间O(n) 空间O(n)

public class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if (root == null) {
            return res;
        }
        boolean flag = true;
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> tem = new ArrayList<Integer>();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
                if (flag) {
                    tem.add(node.val);
                } else {
                    tem.add(0,node.val);
                }
            }
            flag = !flag;
            res.add(tem);
        }
        return res;
    }
}