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]
]

题目大意：给一个二叉树，将这个二叉树以zigzag形式输出，即一行以从左到右的顺序输出，下一行以从右到左的顺序输出……

解题思路：其实本质还是层次遍历，我们可以用cnt记录层数，奇数层从右到左，偶数层从左到右。用一个List记录这一层的数据，从左到右的顺序插入，从右到左的采用头插法插入即可。

Talk is cheap>>

    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        int cnt = 0;
        Deque<TreeNode> queue = new ArrayDeque<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> level = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if ((cnt & 0x1) == 0) {
                    level.add(node.val);
                } else {
                    level.add(0, node.val);
                }
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            res.add(level);
            cnt++;
        }
        return res;
    }