二叉树层序遍历代码输出正确但报NoneType属性错误求助

Hey there! Let's break down why you're seeing that AttributeError: 'NoneType' object has no attribute 'val' error, and how to fix it.

The Root Cause #

The problem hits when you pass an empty tree (i.e., root is None) to your levelOrder method. Here's the play-by-play of what goes wrong:

1. You immediately put root (which is None) into the queue with L.put(root).
2. When you enter the loop, you pull that None node out of the queue with node = L.get().
3. The line local.append(node.val) tries to access the val attribute of a None object—this is exactly what triggers the error on Line 21.

Your existing code already does a good job of checking if node.left and node.right exist before enqueuing them, so non-empty trees would work fine. The only missing piece is handling the empty tree scenario upfront.

The Fix #

All you need to do is add a quick check at the start of the method to handle empty trees. If root is None, just return an empty list right away. Here's the corrected code:

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
import queue
class Solution(object):
    def levelOrder(self, root):
        # Handle empty tree first to avoid enqueuing None
        if not root:
            return []
            
        L = queue.Queue()
        local = []
        result = []
        L.put(root)
        counter = 0
        while not L.empty():
            counter = L.qsize()
            local = []
            while counter > 0:
                node = L.get()
                local.append(node.val)
                if node.left:
                    L.put(node.left)
                if node.right:
                    L.put(node.right)
                counter -= 1
            result.append(local)
        return result

Why This Works #

By checking if not root first, we skip all queue operations when there's no tree to traverse. For non-empty trees, your original logic runs perfectly because you only enqueue valid (non-None) child nodes—so every node you pull from the queue will have a val attribute to access.

内容的提问来源于stack exchange，提问作者anon anon