• 分类：Tree
• 时间复杂度: O(n) 这种把树的节点都遍历一遍的情况时间复杂度为O(n)
• 空间复杂度: O(h) 树的节点的深度

103. 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,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

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

代码：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    
    dict_={}
    
    def zigzagLevelOrder(self, root: 'TreeNode') -> 'List[List[int]]':
        
        self.dict_={}
        
        res=[]
        if root==None:
            return res
        self.helper(root,0)
        i=0
        while i in self.dict_:
            if i%2==1:
                self.dict_[i].reverse()
            res.append(self.dict_[i])
            i+=1
        return res
        
    def helper(self,root,level):
        if root==None:
            return
        if level not in self.dict_:
            self.dict_[level]=[]
        self.dict_[level].append(root.val)
        self.helper(root.left,level+1)
        self.helper(root.right,level+1)

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    
    dict_={}
    
    def zigzagLevelOrder(self, root: 'TreeNode') -> 'List[List[int]]':
        
        self.dict_={}
        
        res=[]
        if root==None:
            return res
        self.helper(root,0)
        i=0
        while i in self.dict_:
            res.append(self.dict_[i])
            i+=1
        return res
        
    def helper(self,root,level):
        if root==None:
            return
        if level not in self.dict_:
            self.dict_[level]=[]
        if level%2==0:
            self.dict_[level].append(root.val)
        else:
            self.dict_[level].insert(0,root.val)
        self.helper(root.left,level+1)
        self.helper(root.right,level+1)

讨论：

1.篮子王和GTH都是用queue做的，不是很看得懂他们写的啥。。。= 。=但是好像用python写挺简单的