根據前序序列和中序序列，重建一顆樹（PHP遞歸實現）

class TreeNode{
	public $data;
	public $lchild = null;
	public $rchild = null;
	public function __construct($data='',$lchild=null,$rchild=null){
		$this->data = $data;
		$this->lchild = $lchild;
		$this->rchild = $rchild;
	}
}

    //根據前序和中序，重建一顆樹
	//$pre 前序遍歷的數組
	//$mid 中序遍歷的數組
	function buildTree($pre,$mid){
		$cnt = count($mid);
		if($cnt<0) return null;
		$root = $pre[0];
		echo '$root==='.$root;
		$node = new TreeNode($root);
		$lenL = 0;
		for($i=0;$i<$cnt;$i++){
			if($mid[$i]== $root){
				$lenL = $i;
				break;
			}
		}
		$lenR = $cnt -$lenL - 1;
		if($lenL>0) $node->lchild = buildTree(array_slice($pre,1,$lenL),array_slice($mid,0,$lenL));
		if($lenR>0) $node->rchild = buildTree(array_slice($pre,$lenL+1,$lenR),array_slice($mid,$lenL+1,$lenR));
		return $node;
	}
	
	$mid = array(4,7,2,1,5,3,8,6);
	$pre = array(1,2,4,7,3,5,6,8);
	$node = buildTree($pre,$mid);
	echo '<pre>';
	var_dump($node);
	echo '</pre>';