lc102.二叉树的层序遍历

class Solution:

    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        if not root:
            return []
        queue, res = [root], []
        while queue:
            tmp = []
            for _ in range(len(queue)):
                node = queue.pop(0)
                tmp.append(node.val)
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
            res.append(tmp)
        return res