PHP实现二叉树遍历(非递归方式，栈模拟实现)

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：



对于二叉树的以前都是用C写的，递归遍历， 用PHP也可以简单模拟，下面这个例子算是最简单的一种遍历了（参考网络的）。

/**
* 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
preorder($a);  //结果是：A B D C E F
inorder($a);  //结果是： D B A E C F
postorder($a); //结果是:  D B E F C A

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