$arr = array(
    array(
        'id' => 1,
        'parentid' => 0,
        'title' => 'a'
    ),
    array(
        'id' => 2,
        'parentid' => 1,
        'title' => 'a-1'
    ),
    array(
        'id' => 3,
        'parentid' => 1,
        'title' => 'a-1'
    ),
    array(
        'id' => 4,
        'parentid' => 2,
        'title' => 'a-1-1'
    )  
);
递归转换为数组如下:
$arr = array(
    array(
        'id' => 1,
        'parentid' => 0,
        'title' => 'a',
        'child' => array(
                       array(
                         'id' => 2,
                         'parentid' => 1,
                         'title' => 'a-1',
                         'child' => array(
                                    ......
                                    ) 
                       array(
                         'id' => 3,
                         'parentid' => 2,
                         'title' => 'a-2'
                       )
    ), 
    ......
);
请问要如何写这个递归呢?
回复讨论(解决方案) function findchildren($list, $p=0){ $r = array(); foreach($list as $id=>$item){ if($item['parentid'] == $p) { $r[$id] = $item; if($t = findchildren($list, $item['id']) ) $r[$id]['children'][] = $t; } } return $r;} $arr = array( array( 'id' => 1, 'parentid' => 0, 'title' => 'a' ), array( 'id' => 2, 'parentid' => 1, 'title' => 'a-1' ), array( 'id' => 3, 'parentid' => 1, 'title' => 'a-1' ), array( 'id' => 4, 'parentid' => 2, 'title' => 'a-1-1' ) );print_r(findchildren($arr));
array
(
    [0] => array
        (
            [id] => 1
            [parentid] => 0
            [title] => a
            [children] => array
                (
                    [0] => array
                        (
                            [1] => array
                                (
                                    [id] => 2
                                    [parentid] => 1
                                    [title] => a-1
                                    [children] => array
                                        (
                                            [0] => array
                                                (
                                                    [3] => array
                                                        (
                                                            [id] => 4
                                                            [parentid] => 2
                                                            [title] => a-1-1
                                                        )
)
)
)
[2] => array
                                (
                                    [id] => 3
                                    [parentid] => 1
                                    [title] => a-1
                                )
)
)
)
)