94. 二叉树前中后序遍历

<?php
Class TreeNode {
    /** @var int $val */
    public $val = 0;
    /** @var TreeNode $left */
    public $left;
    /** @var TreeNode $right */
    public $right;
    public function __construct($val) {
        $this->val = $val;
    }
}
class Solution {
    public array $res = [];
    public function preorderTraversal($root) {
        if ($root) {
            array_push($this->res, $root->val);
            $this->inorderTraversal($root->left);
            $this->inorderTraversal($root->right);
        }
        return $this->res;
    }
    public function inorderTraversal($root) {
        if ($root) {
            $this->inorderTraversal($root->left);
            array_push($this->res, $root->val);
            $this->inorderTraversal($root->right);
        }
        return $this->res;
    }
    public function postorderTraversal($root) {
        if ($root) {
            $this->inorderTraversal($root->left);
            $this->inorderTraversal($root->right);
            array_push($this->res, $root->val);
        }
        return $this->res;
    }
}
/**
 *          1
 *         / \
 *        2   3
 *       / \  /
 *      4  5  6
 */
$tree = new TreeNode(1);
$tree->left = new TreeNode(2);
$tree->left->left = new TreeNode(4);
$tree->left->right = new TreeNode(5);
$tree->right = new TreeNode(3);
$tree->right->left = new TreeNode(6);
$cls = new Solution();
$r = $cls->preorderTraversal($tree);
echo '前序遍历：' . implode(',', $r) . "\n";
$cls1 = new Solution();
$r = $cls1->inorderTraversal($tree);
echo '中序遍历：' . implode(',', $r) . "\n";
$cls2 = new Solution();
$r = $cls2->postorderTraversal($tree);
echo '后序遍历：' . implode(',', $r) . "\n";