[leetcode tree]102. Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

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

按层打印树

 class Solution(object):
     def levelOrder(self, root):
         res,level = [],[root]
         while root and level:
             res.append([node.val for node in level])
             level_pair = [(node.left,node.right) for node in level]
             level = [leaf for lr in level_pair for leaf in lr if leaf]
         return res