LeetCode-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).

For example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

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

题意：给定一颗二叉树，要求输出z字型的层序遍历；

解法：对于层次遍历，可以想到利用队列来实现；那么，对于题目要求的z字型的层次遍历，需要额外一个变量，用来记录在每一层中的遍历方向；我们用链表来存储每一层的节点，这样，当层的遍历为从左至右的时候，我们向链表的尾部插入元素；当层的遍历为从右至左的时候，我们向链表的首部插入元素；

Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List> zigzagLevelOrder(TreeNode root) {
        ArrayList> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        LinkedList queue = new LinkedList<>();
        queue.addLast(root);
        int len = 1;
        int direction = 1;
        while (!queue.isEmpty()) {
            LinkedList list = new LinkedList<>();
            int temp = len;
            len = 0;
            while (temp-- > 0) {
                TreeNode node = queue.poll();
                if (direction == 1) {
                    list.addLast(node.val);
                } else {
                    list.addFirst(node.val);
                }
                if (node.left != null) {
                    queue.addLast(node.left);
                    len++;
                }
                if (node.right != null) {
                    queue.addLast(node.right);
                    len++;
                }
            }
            result.add(list);
            direction *= -1;
        }

        return result;
    }
}