public class Solution
extends Object
145 - Binary Tree Postorder Traversal\.
Easy
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`
**Follow up:** Recursive solution is trivial, could you do it iteratively?