二叉树及其变体是数据结构家族里的重要组成部分。最为链表的一种变体，二叉树最适合处理需要一特定次序快速组织和检索的数据。
代码如下:
<?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>';
?>
