[Leet Code] Binary Tree Postorder Traversal

Leetcode: https://leetcode.com/problems/binary-tree-postorder-traversal/

Problem:

Given the root of a binary tree, return the postorder traversal of its nodes' values.

Example 1:

Input: root = [1,null,2,3]
Output: [3,2,1]

Example 2:

Input: root = []
Output: []

Example 3:

Input: root = [1]
Output: [1]

Example 4:

Input: root = [1,2]
Output: [2,1]

Example 5:

Input: root = [1,null,2]
Output: [2,1]

Constraints:

• The number of the nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100

Solution:

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution(object):
    def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        listOfElements = []
        self.postorder(root, listOfElements)
        return listOfElements
        
    def postorder(self, root, listOfElements):
        if root:
            self.postorder(root.left, listOfElements)
            self.postorder(root.right, listOfElements)
            listOfElements.append(root.val)

Explanation:

As we go through the tree in postorder (left, right, root) we append the values to the list.