# 103. Binary Tree Zigzag Level Order Traversal(Medium)
###### tags: `BFS`, `Binary Tree`, `Bloomberg`
[Link](https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/)
Description:
Given the `root` of a binary tree, return the *zigzag level order traversal of its nodes' values*. (i.e., from left to right, then right to left for the next level and alternate between).

**Solution**
* BFS

class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        Queue<Integer> q = new LinkedList<>();
        q.offer(root);
        boolean reverse = false;
        while (!q.isEmpty()) {
            int size = q.size();
            List<Integer> list = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode node = q.poll();
                if (node.left != null) q.offer(node.left);
                if (node.right != null) q.offer(node.right);

                if (!reverse) list.add(node.val);
                else list.add(0, node.val);
            }
            res.add(list);
            reverse = !reverse;
        }
        return res;
    }
}

Time Complexity: O(N)
Space Complexity: O(N)