PHP implementation of an infinite class of classification problem

today, a netizen has a problem:

give the following parent-child structure (you can describe it in a similar structure in your language, the first column is the parent, the second column is the child), and comb it into a similar hierarchical parent-child structure. Origin= [('A112','A1122'),          ('A','A1'),          ('A','A2'),          ('A1','A11'),          ('A2','A21'),          ('A2','A22'),          ('A','A3'),          ('A22','A221'),          ('A11','A111'),          ('A21','A211'),          ('A11','A112'),          ('A21','A212'),          ('A11','A113'),          ('A112','A1121'),          ('A3','A31'),          ('A31','A311'),          ('A22','A222'),          ('A31','A312'),          ('A31','A313'),          ('A311','A3111'),          ('A312','A3121'),          ('A3111','A31111')]

Output result:

Look at the principle of infinite class classification, can be implemented by recursion:

<?PHP$origin= [    [' A112 ', ' A1122 '],  [' A ', ' A1 '],  [' A ', ' A2 '],  [' A1 ', ' A11 '],  [' A2 ', ' A21 '],  [' A2 ', ' A22 '],  [' A ', ' A3 '],  [' A22 ', ' A221 '],  [' A11 ', ' A111 '],  [' A21 ', ' A211 '],  [' A11 ', ' A112 '],  [' A21 ', ' A212 '],  [' A11 ', ' A113 '],  [' A112 ', ' A1121 '],  [' A3 ', ' A31 '],  [' A31 ', ' A311 '],  [' A22 ', ' A222 '],  [' A31 ', ' A312 '],  [' A31 ', ' A313 '],  [' A311 ', ' A3111 '],  [' A312 ', ' A3121 '],  [' A3111 ', ' A31111 ']];//find child information based on parent IDfunctionLevel$cate,$html= '-',$pid= ' 0 ',$level= 0) {    $arr=Array(); foreach($cate  as $val) {        if($val[0] = =$pid) {            $val[' level '] =$level+ 1; $val[' HTML '] =str_repeat($html,$level); $arr[] =$val; $arr=Array_merge($arr, Level ($cate,$html,$val[1],$level+ 1)); }    }    return $arr;}//Building top-level classifications$pids= [];$cids= [];foreach($origin  as $val) {    $pids[] =$val[0]; $cids[] =$val[1];}$top=Array_flip(Array_diff($pids,$cids));foreach($top  as $key=$val) {    $origin[] = [' 0 ',$key];}$res= Level ($origin, ‘ ‘);foreach($res  as $k=$v) {    Echo $v[' HTML '],$v[1],Php_eol;}

Output:

PHP implementation of an infinite class of classification problem