Binary Tree Preorder Traversal issue
Lanehesser
72

I'm looking for some help with the Preorder Traversal problem. This is the code that I came up with. It passes test one (Input: [1, null, 2, 3], Output: [1, 2, 3]), but fails test two (Input: [], Output: [1, 2, 3], Expected: [])

class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution:
	def preorderTraversal(self, root: TreeNode, result=[]):
		if root:
			result.append(root.val)
			self.preorderTraversal(root.left, result)
			self.preorderTraversal(root.right, result)
		return result

However, I found the code below that another user had come up with and it passes all the tests.

class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution:
    def transverse(self, node, content):
        if node: 
            content.append(node.val)
            self.transverse(node.left, content)
            self.transverse(node.right, content)
    
        return content
    
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        return self.transverse(root, [])

To me, these two solutions look the same and when I test them, I get the same output, so I don't understand why my code is failing. If anyone could help, I would appreciate it.

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