利用PHP的引用生成树的结构

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PHP中的引用就是两个变量指向了同一个地方，只要在变量前面增加了&符号，它就变成了一个引用

$a='aaa';
$b=&$a;
$c=&$b;
xdebug_debug_zval('a','b','c');

输出结果是：

a: (refcount=3, is_ref=1)='aaa'
b: (refcount=3, is_ref=1)='aaa'
c: (refcount=3, is_ref=1)='aaa'

“aaa”有了三个引用 ， 并且是is_ref是引用类型，那也就意味着不管是我修改$b ，还是修改$c , "aaa"这个都会被改变

可以根据上面这个原理来把一个数据库存储的带pid的逐行数据，变成一个多层级的树状结构

$data=array(
array("id"=>2,"pid"=>1),
array("id"=>3,"pid"=>1),
array("id"=>4,"pid"=>2),
array("id"=>5,"pid"=>2),
array("id"=>6,"pid"=>3),
array("id"=>7,"pid"=>3),
array("id"=>1,"pid"=>0),
);
$refer=array();//存储主键与数组单元的引用关系
//遍历
foreach($data as $k=>$v){
$refer[$v['id']]=&$data[$k];//为每个数组成员建立对应关系
}
//遍历2
foreach($data as $k=>$v){
$parent=&$refer[$v['pid']];//获取父分类的引用
$parent['child'][]=&$data[$k];//在父分类的children中再添加一个引用成员
}
print_r($data);

利用了一个$refer数组，时间复杂度是O(n) , 只需要单层循环，直接通过引用修改$data原数据，生成一个树状结构

Array
(
[0] => Array
(
[id] => 2
[pid] => 1
[child] => Array
(
[0] => Array
(
[id] => 4
[pid] => 2
)

[1] => Array
(
[id] => 5
[pid] => 2
)

)

)

[1] => Array
(
[id] => 3
[pid] => 1
[child] => Array
(
[0] => Array
(
[id] => 6
[pid] => 3
)

[1] => Array
(
[id] => 7
[pid] => 3
)

)

)

[2] => Array
(
[id] => 4
[pid] => 2
)

[3] => Array
(
[id] => 5
[pid] => 2
)

[4] => Array
(
[id] => 6
[pid] => 3
)

[5] => Array
(
[id] => 7
[pid] => 3
)

[6] => Array
(
[id] => 1
[pid] => 0
[child] => Array
(
[0] => Array
(
[id] => 2
[pid] => 1
[child] => Array
(
[0] => Array
(
[id] => 4
[pid] => 2
)

[1] => Array
(
[id] => 5
[pid] => 2
)

)

)

[1] => Array
(
[id] => 3
[pid] => 1
[child] => Array
(
[0] => Array
(
[id] => 6
[pid] => 3
)

[1] => Array
(
[id] => 7
[pid] => 3
)

)

)

)

)

)

$a='aaa';

$b=&$a;

$c=&$b;

xdebug_debug_zval('a','b','c');