[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]
]

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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