PHP Class&Object -- 解析PHP实现二叉树

<?php
// define a class to implement a binary tree
class binary_tree_node {
    // define the variable to hold our data:
    public $data;
    // and a variable to hold the left and right objects:
    public $left;
    public $right;

    // a constructor method that allows for data to be passed in
    public function __construct($d = null) {
        $this->data = $d;
    }

    // traverse the tree, left to right, in pre-order, returning an array
    // preorder means that each node's value preceeds its children.
    public function traversepreorder() {
        // prep some variables.
        $l = array();
        $r = array();
        // read in the left and right children appropriately traversed:
        if ($this->left) { $l = $this->left->traversepreorder(); }
        if ($this->right) { $r = $this->right->traversepreorder(); }

        // return a merged array of the current value, left, and right:
        return array_merge(array($this->data), $l, $r);
    }
    // traverse the tree, left to right, in postorder, returning an array
    // postorder means that each node's value follows its children.
    public function traversepostorder() {
        // prep some variables.
        $l = array();
        $r = array();
        // read in the left and right children appropriately traversed:
        if ($this->left) { $l = $this->left->traversepostorder(); }
        if ($this->right) { $r = $this->right->traversepostorder(); }

        // return a merged array of the current value, left, and right:
        return array_merge($l, $r, array($this->data));
    }
    // traverse the tree, left to right, in-order, returning an array.
    // in-order means that values are ordered as left children, then the
    //  node value, then the right children.
    public function traverseinorder() {
        // prep some variables.
        $l = array();
        $r = array();
        // read in the left and right children appropriately traversed:
        if ($this->left) { $l = $this->left->traverseinorder(); }
        if ($this->right) { $r = $this->right->traverseinorder(); }

        // return a merged array of the current value, left, and right:
        return array_merge($l, array($this->data), $r);
    }
}
// let's create a binary tree that will equal the following:    3
//                                                             / /     
//                                                            h   9     
//                                                               / /    
// create the tree:                                             6   a   
$tree = new binary_tree_node(3);
$tree->left = new binary_tree_node('h');
$tree->right = new binary_tree_node(9);
$tree->right->left = new binary_tree_node(6);
$tree->right->right = new binary_tree_node('a');
// now traverse this tree in all possible orders and display the results:
// pre-order: 3, h, 9, 6, a
echo '<p>', implode(', ', $tree->traversepreorder()), '</p>';
// post-order: h, 9, 6, a, 3
echo '<p>', implode(', ', $tree->traversepostorder()), '</p>';
// in-order: h, 3, 6, 9, a
echo '<p>', implode(', ', $tree->traverseinorder()), '</p>';
?>