Binary Tree Postorder Traversal
Difficulty: Easy
Category: DSA
Topics: Stack, Tree, Depth-First Search, Binary Tree
Asked at: Amazon
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]
**Explanation:**
**Example 2:**
**Input:** root = [1,2,3,4,5,null,8,null,null,6,7,9]
**Output:** [4,6,7,5,2,9,8,3,1]
**Explanation:**
**Example 3:**
**Input:** root = []
**Output:** []
**Example 4:**
**Input:** root = [1]
**Output:** [1]
**Constraints:**
- The number of the nodes in the tree is in the range `[0, 100]`.
- `-100 <= Node.val <= 100`
**Follow up:** Recursive solution is trivial, could you do it iteratively?