## Tree Node Definition#

``````# Definition for a binary tree node.
from typing import List

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

## Recursive Implementation#

### `root` ➡️ `left` ➡️ `right`#

``````class Solution:
def traversal(self, root: 'TreeNode', ans=None) -> List[List[int]]:
"""
0
/     \
1        2
/   \
3       4
/   \
6       9

Pre-order...: [0, 1, 3, 4, 5, 9, 2]
"""
if not root:
return []

if ans is None:
ans = []

ans.append(root.val)        # Pre-order Traversal
self.traversal(root.left, ans=ans)
self.traversal(root.right, ans=ans)

return ans
``````

## Iterative Implementation#

`root` ➡️ `left` ➡️ `right`

``````class Solution:
def traversal(self, root: 'TreeNode')->List[List[int]]:
"""
0
/     \
1        2
/   \
3       4
/   \
6       9

Pre-order...: [0, 1, 3, 4, 5, 9, 2]
"""

if not root:
return root

stack = [root]
results = []

while stack:
node = stack.pop()

if node:
results.append(node.val)

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

return results
``````