PHP 树的先序,中序,后序遍历

<?php
    #树节点
    class Node {
        public $lc = null;    #左孩子
        public $rc = null;    #右孩子
        public $val;
        
        public function __construct($val) {
            $this->val = $val;
        }
    }
    
    #先序遍历
    function preTraverse(Node $root) {
        echo $root->val . " ";
        if($root->lc != null) preTraverse($root->lc);
        if($root->rc != null) preTraverse($root->rc);
    }
    
    #中序遍历
    function inTraverse(Node $root) {
        if($root->lc != null) inTraverse($root->lc);
        echo $root->val . " ";
        if($root->rc != null) inTraverse($root->rc);
    }
    
    #后序遍历
    function postTraverse(Node $root) {
        if($root->lc != null) postTraverse($root->lc);
        if($root->rc != null) postTraverse($root->rc);
        echo $root->val . " ";
    }
    
    $root = new Node(3);
    $node1 = new Node(2);
    $node2 = new Node(1);
    $node3 = new Node(6);
    $node4 = new Node(5);
    $node5 = new Node(7);
    
    $root->lc = $node1;
    $root->rc = $node3;
    $node1->lc = $node2;
    $node3->lc = $node4;
    $node3->rc = $node5;
    
    preTraverse($root);
    echo "<br>";
    inTraverse($root);
    echo "<br>";
    postTraverse($root);
?>

输出

3 2 1 6 5 7 
1 2 3 5 6 7 
1 2 5 7 6 3