php实现先序、中序、后序遍历二叉树

二叉树是每个节点最多有两个子树的树结构。通常子树被称作“左子树”（left subtree）和“右子树”（right subtree）。二叉树常被用于实现二叉查找树和二叉堆

<?php
class Node{
    public $value;
    public $left;
    public $right;
}
//先序遍历 根节点 ---> 左子树 ---> 右子树
function preorder($root){
    $stack=array();
    array_push($stack,$root);
    while(!empty($stack)){
        $center_node=array_pop($stack);
        echo $center_node->value.' ';//先输出根节点
        if($center_node->right!=null){
            array_push($stack,$center_node->right);//压入左子树
        }
        if($center_node->left!=null){
            array_push($stack,$center_node->left);
        }
    }
}
//中序遍历，左子树---> 根节点 ---> 右子树
function inorder($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 = $center_node->right;
         }
}
//后序遍历，左子树 ---> 右子树 ---> 根节点
function tailorder($root){
    $stack=array();
    $outstack=array();
    array_push($stack,$root);
    while(!empty($stack)){
        $center_node=array_pop($stack);
        array_push($outstack,$center_node);//最先压入根节点，最后输出
        if($center_node->left!=null){
            array_push($stack,$center_node->left);
        }
        if($center_node->right!=null){
            array_push($stack,$center_node->right);
        }
    }
    
    while(!empty($outstack)){
        $center_node=array_pop($outstack);
        echo $center_node->value.' ';
    }
    
}
$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;
preorder($a);//A B D C E F 
echo '<hr/>';
inorder($a);//D B A E C F
echo '<hr/>';
tailorder($a);//D B E F C A

结果：

A B D C E F

---

D B A E C F

---

D B E F C A