145. Binary Tree Postorder Traversal

Edited by Charles

Easy · Python · Binary Tree

Problem

Read the original problem on LeetCode.

The official page contains the problem statement, examples, and constraints.

Explanation, examples & other approaches

Core idea

Reference to 144. Binary Tree Preorder Traversal

Original study notes follow, including intermediate snippets and earlier attempts.

Reference to 144. Binary Tree Preorder Traversal

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def postorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        if not root:
            return []

        stack = [root]
        result = []

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

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

        return result[::-1]

Study record

Field	Value
Attempts	1
Mastery	❚
The Latest Attempt	January 12, 2026

Python solution

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:

        def preorder(node):
            if node is None:
                return

            # 写在这里是Preorder
            # res.append(node.val)

            preorder(node.left)

            # Inorder
            # res.append(node.val)

            preorder(node.right)

            # Postorder
            res.append(node.val)

        res = []
        preorder(root)
        return res

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