Given the root of a binary tree
root, return the node values in depth-first order: left — right — root (postorder).Example 1
Input: root = [1,null,2,3]
Output: [3,2,1]
Explanation: Postorder: left, right, then root.
Output: [3,2,1]
Explanation: Postorder: left, right, then root.
Example 2
Input: root = [1]
Output: [1]
Explanation: A tree with one node.
Output: [1]
Explanation: A tree with one node.