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#

`left` ➡️ `right` ➡️ `root`#

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

Post-order...: [3, 6, 9, 4, 1, 2, 0]

T: O(N)
S: O(N) worst case; O(log N) average
"""

if result is None:
result = []

if not root:
return []

self.traversal(root.left, result=result)
self.traversal(root.right, result=result)
result.append(root.val)

return result
``````

Iterative Implementation#

`left` ➡️ `right` ➡️ `root`#

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

Post-order...: [3, 6, 9, 4, 1, 2, 0]

T: O(N)
S: O(N)
"""

if not root:
return []

stack = [(root, False)]
result = []

while stack:
node, visited = stack.pop()

if node:
if visited:
result.append(node.val)
else:
stack.append((node, True))
stack.append((node.right, False))
stack.append((node.left, False))

return result

``````