最近遇到这样一个问题：

说实话，一开始没想到怎么写出来，然后当天晚上回来后研究了下。答案如下； 

虽然是完成了，但总觉得代码多余，经过几次修改后如下：

。也想过不用数组降维的方式实现，目前没有好的思路。先这样了。代码贴出来：

function listToTree($arr){

$tree = []; 

$data = [];

//数组降维  增加pid

foreach($arr as $k => $v){

$level = count($v) / 2 + 1;

for($i = 1; $i <= $level; $i++){

if(isset($v[$i.'_class'])){

$item[$v[$i.'_id']]['value']    = $v[$i.'_id'];

$item[$v[$i.'_id']]['label']    = $v[$i.'_class'];

$item[$v[$i.'_id']]['pid']      = isset($v[($i - 1).'_id']) ? $v[($i - 1).'_id'] : 0;

$item[$v[$i.'_id']]['children'] = [];

$data[$v[$i.'_id']] = $item[$v[$i.'_id']];

}

}

}

//ksort($data);

foreach($data as $key => $item){

$parentId = $item['pid'];

unset($data[$key]['pid']);

if(0 == $parentId){

$tree[] = &$data[$key];

}else{

if(isset($data[$parentId])){

$parent = &$data[$parentId];

$parent['children'][] = &$data[$key];

}

}

}

return $tree;

}

数组：$arr = [

[

'1_class' => '工具',

'2_class' => '备忘录',

'1_id' => 1,

'2_id' => 2,

],

[

'1_class' => '教育',

'2_class' => '学历教育',

'3_class' => '中等',

'1_id' => 3,

'2_id' => 4,

'3_id' => 6,

],

[

'1_class' => '教育',

'2_class' => '学历教育',

'3_class' => '高等',

'1_id' => 3,

'2_id' => 4,

'3_id' => 5,

],

[

'1_class' => '教育',

'2_class' => '成人教育',

'1_id' => 3,

'2_id' => 7,

],

];

结果：[{

"value": 1,

"label": "工具",

"children": [{

"value": 2,

"label": "备忘录",

"children": []

}]

}, {

"value": 3,

"label": "教育",

"children": [{

"value": 4,

"label": "学历教育",

"children": [{

"value": 6,

"label": "中等",

"children": []

}, {

"value": 5,

"label": "高等",

"children": []

}]

}, {

"value": 7,

"label": "成人教育",

"children": []

}]

}]