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?