[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

 class Solution(object):
     def zigzagLevelOrder(self, root):
         res,level = [],[root]
         flag = 0
         while root and level:
             if not flag:
                 res.append([r.val for r in level])
             else:
                 res.append([r.val for r in level[::-1]])
             flag = 1-flag
             pair = [(v.left,v.right) for v in level]
             level = [v for lr in pair for v in lr if v]
         return res

改：

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