leetcode-103. Binary Tree Zigzag Level Order Traversal

题目：

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]
]

和上一题层序遍历很类似，不过要求是Z字型遍历，所以这里用栈，而且需要一个bollean变量来表示方向。

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {        List<List<Integer>> ret = new ArrayList<List<Integer>>();        Stack<TreeNode> tmp = new Stack<TreeNode>();        if(root!=null) tmp.add(root);        boolean flag = true;        while(tmp.size()!=0){            List<Integer> ad = new ArrayList<Integer>();            Stack<TreeNode> replace = new Stack<TreeNode>();            while(tmp.size()!=0){                TreeNode node = tmp.pop();                if(!flag){                    if(node.right!=null)replace.add(node.right);                    if(node.left!=null)replace.add(node.left);                    ad.add(node.val);                }else{                    if(node.left!=null)replace.add(node.left);                    if(node.right!=null)replace.add(node.right);                    ad.add(node.val);                }            }            flag = !flag;            tmp = replace;            ret.add(ad);        }        return ret;    }}