<?php
class Node {
  public $value;
  public $child_left;
  public $child_right;
}
final class Ergodic {
  //前序遍历:先访问根节点，再遍历左子树，最后遍历右子树；并且在遍历左右子树时，仍需先遍历根节点，然后访问左子树，最后遍历右子树
  public static function preOrder($root){
    $stack = array();
    array_push($stack, $root);
    while(!empty($stack)){
      $center_node = array_pop($stack);
      echo $center_node->value . ' ';
      //先把右子树节点入栈，以确保左子树节点先出栈
      if($center_node->child_right != null) array_push($stack, $center_node->child_right);
      if($center_node->child_left != null) array_push($stack, $center_node->child_left);
    }
  }
  //中序遍历:先遍历左子树、然后访问根节点，最后遍历右子树；并且在遍历左右子树的时候。仍然是先遍历左子树，然后访问根节点，最后遍历右子树
  public static function midOrder($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->child_left;
      }
      $center_node = array_pop($stack);
      echo $center_node->value . ' ';
      $center_node = $center_node->child_right;
    }
  }
  //后序遍历:先遍历左子树，然后遍历右子树，最后访问根节点；同样，在遍历左右子树的时候同样要先遍历左子树，然后遍历右子树，最后访问根节点
  public static function endOrder($root){
    $push_stack = array();
    $visit_stack = array();
    array_push($push_stack, $root);
    while (!empty($push_stack)) {
      $center_node = array_pop($push_stack);
      array_push($visit_stack, $center_node);
      //左子树节点先入$pushstack的栈，确保在$visitstack中先出栈
      if ($center_node->child_left != null) array_push($push_stack, $center_node->child_left);
      if ($center_node->child_right != null) array_push($push_stack, $center_node->child_right);
    }
    while (!empty($visit_stack)) {
      $center_node = array_pop($visit_stack);
      echo $center_node->value . ' ';
    }
  }
}
//创建二叉树
$a = new Node();
$b = new Node();
$c = new Node();
$d = new Node();
$e = new Node();
$f = new Node();
$g = new Node();
$h = new Node();
$i = new Node();
$a->value = 'A';
$b->value = 'B';
$c->value = 'C';
$d->value = 'D';
$e->value = 'E';
$f->value = 'F';
$g->value = 'G';
$h->value = 'H';
$i->value = 'I';
$a->child_left = $b;
$a->child_right = $c;
$b->child_left = $d;
$b->child_right = $g;
$c->child_left = $e;
$c->child_right = $f;
$d->child_left = $h;
$d->child_right = $i;
//前序遍历
Ergodic::preOrder($a); //结果是：A B D H I G C E F
echo '<br/>';
//中序遍历
Ergodic::midOrder($a); //结果是： H D I B G A E C F
echo '<br/>';
//后序遍历
Ergodic::endOrder($a); //结果是: H I D G B E F C A