Binary Tree Preorder Traversal

Edge Case: When the input tree is empty (root == None), pushing it directly onto the stack will cause an error.
To handle this, add an early check:
if not root: return [].

The correct code would be :

class Solution:
def preorderTraversal(self, root):
if not root:
return []

result = []
stack = [root]

while stack:
node = stack.pop()
result.append(node.val)

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

return result

Видео Binary Tree Preorder Traversal канала Gokul Krishna B