Binary Tree Postorder Traversal

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

Class TreeNode:
    val (int)
    left (TreeNode)
    right (TreeNode)

Input Format

First Parameter - TreeNode root

Output Format

Return the array of values of nodes in postorder sequence

Example 1:

Input: 
    1 2 3 7 4 null 8
Output: 
    7 4 2 8 3 1

Example 2:

Input:
    1 null 2 null 5 3 8 null 4
Output: 
    4 3 8 5 2 1

Constraints

• The number of the nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100
• Expected Time Complexity: O(n)
• Expected Space Complexity: O(1)