Binary Tree Level Order Traversal——Breadth-first Search

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]
]

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        s = []
        nodes = []
        if not root: return s
        nodes.append(root)
        while nodes:
        	s.append([n.val for n in nodes])
        	tmp = []
        	for node in nodes:
        		if node.left: tmp.append(node.left)
        		if node.right: tmp.append(node.right)
        	nodes = tmp
        return s