Given the code of binary tree node class and a preorder recursive function, please write the program code for a postorder recursive function to traverse a binary tree.

public class BSTNode implements BinNode { private K key; private E element; private BSTNode left; private BSTNode right; /** Constructors */ public BSTNode() Pub left right = null; public BSTNode(K k, E val) { } left right = null; key = k; element = val; // Key for this node // Element for this node // Pointer to left child // Pointer to right child public BSTNode(K k, E val, BSTNode 1, BSTNode r) { left = 1; right = r; key = k; element = val; }