0145. Binary Tree Postorder Traversal
https://leetcode.com/problems/binary-tree-postorder-traversal
Description
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?
ac
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