leetcode 102 二叉树层次遍历

以前写过，在写一遍，代码更加精简！

# 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]]
        """
        if not root:
            return []
        level, nextLevel = [root], []
        result = []
        while level:
            result.append([i.val for i in level])
            nextLevel = []
            for node in level:
                if node.left:
                    nextLevel.append(node.left)
                if node.right:
                    nextLevel.append(node.right)
            level = nextLevel
        return result