[LeetCode]Binary Tree Zigzag Level Order Traversal

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]]

bfs+递归 思路基本同   Binary Tree Level Order Traversal

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {List<List<Integer>> res = new ArrayList<>();public List<List<Integer>> zigzagLevelOrder(TreeNode root) {if (root == null) return res;List<TreeNode> list = new ArrayList<TreeNode>();list.add(root);levelOrder(list, true);return res;}private void levelOrder(List<TreeNode> list, boolean boo) {if (list.size() == 0)return;List<TreeNode> tn = new ArrayList<TreeNode>();List<Integer> it = new ArrayList<>();for (int i = 0; i < list.size(); i++) {TreeNode treeNode = list.get(i);if(boo){    it.add(list.get(i).val);}else{    it.add(list.get(list.size()-i-1).val);}if (treeNode.left != null) {tn.add(list.get(i).left);}if (treeNode.right != null) {tn.add(list.get(i).right);}}res.add(it);levelOrder(tn, !boo);}}