[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