/**
 * 二叉树遍历
 * @blog
 */
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