Leetcode :面试题32

用一个队列存放每一层的节点， 遍历该队列再把下一层的节点存放到队列，

循环终止：队列为空

# 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[int]
        """
        if root is None:
            return []
        r = [root]
        out=[]
        while len(r)!=0:
            b=[]
            for node in r:
                out.append(node.val)
                if node.left is not None:
                    b.append(node.left)
                if node.right is not None:
                    b.append(node.right)
            r = b
        return out