[leetcode tree]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]
]

之字形层序遍历树

思路：设立一个标志位，当这个标志位为0时候，从左到右打印，当标志位为1时候，从右往左打印，每遍历一层，标志位变一次

 1 class Solution(object):
 2     def zigzagLevelOrder(self, root):
 3         res,level = [],[root]
 4         flag = 0
 5         while root and level:
 6             if not flag:
 7                 res.append([r.val for r in level])
 8             else:
 9                 res.append([r.val for r in level[::-1]])
10             flag = 1-flag
11             pair = [(v.left,v.right) for v in level]
12             level = [v for lr in pair for v in lr if v]
13         return res

改：

1 class Solution(object):
2     def zigzagLevelOrder(self, root):
3         res,level,flag= [],[root],1
4         while root and level:
5             res.append([r.val for r in level[::flag]])
6             flag *= -1
7             pair = [(v.left,v.right) for v in level]
8             level = [v for lr in pair for v in lr if v]
9         return res