leetcode 513 find bottom left tree value

class (object):
    def findBottomLeftValue(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        
        Q = collections.deque([root,None])
        lastnode,res = None,0
        while Q:
            node = Q.popleft()
            if lastnode == None:
                res = node.val
            lastnode = node
            # 处理一层结束时的情况
            if node == None:
                if not Q:
                    return res
                Q.append(None)
                continue

            if node.left: Q.append(node.left)
            if node.right: Q.append(node.right)

74 / 74 test cases passed.
difficulty: medium
Runtime: 36 ms,beats 100%