php 实现（非递归，栈模拟）二叉树的先序、中序、后序

二叉树定义是这样的：一棵非空的二叉树由根结点及左、右子树这三个基本部分组成，根据节点的访问位置不同有三种遍历方式：
① NLR：前序遍历(PreorderTraversal亦称(先序遍历))
——访问结点的操作发生在遍历其左右子树之前。
② LNR：中序遍历(InorderTraversal)
——访问结点的操作发生在遍历其左右子树之中(间)。
③ LRN：后序遍历(PostorderTraversal)
——访问结点的操作发生在遍历其左右子树之后。
如下图：

代码如下：

<?php/** * 二叉树遍历 * @blog<http://www.phpddt.com> */class Node {    public $value;    public $left;    public $right;}//前序遍历，访问根节点->遍历子左树->遍历右左树function preorder($root){    $stack = array();    array_push($stack, $root);    while(!empty($stack)){        $center_node = array_pop($stack);        echo $center_node->value.' ';        if($center_node->right != null) array_push($stack, $center_node->right);        if($center_node->left != null) array_push($stack, $center_node->left);    }}//中序遍历，遍历子左树->访问根节点->遍历右右树function inorder($root){    $stack = array();    $center_node = $root;    while (!empty($stack) || $center_node != null) {             while ($center_node != null) {                 array_push($stack, $center_node);                 $center_node = $center_node->left;             }             $center_node = array_pop($stack);             echo $center_node->value . " ";             $center_node = $center_node->right;         }}//后序遍历，遍历子左树->访问子右树->遍历根节点function postorder($root){        $pushstack = array();        $visitstack = array();        array_push($pushstack, $root);        while (!empty($pushstack)) {            $center_node = array_pop($pushstack);            array_push($visitstack, $center_node);            if ($center_node->left != null) array_push($pushstack, $center_node->left);            if ($center_node->right != null) array_push($pushstack, $center_node->right);        }        while (!empty($visitstack)) {            $center_node = array_pop($visitstack);            echo $center_node->value." ";        }}//创建如上图所示的二叉树$a = new Node();$b = new Node();$c = new Node();$d = new Node();$e = new Node();$f = new Node();$a->value = 'A';$b->value = 'B';$c->value = 'C';$d->value = 'D';$e->value = 'E';$f->value = 'F';$a->left = $b;$a->right = $c;$b->left = $d;$c->left = $e;$c->right = $f;//前序遍历echo '前序遍历：';preorder($a);  //结果是：A B D C E Fecho '<br>';echo '中序遍历：';inorder($a);  //结果是： D B A E C Fecho '<br>';echo '后序遍历：';postorder($a); //结果是:  D B E F C Aecho '<br>';?>