Convert the list containing the parent ID to a tree, and the id list to a tree _ PHP Tutorial

Convert the list containing the parent ID into a tree, and the id list into a tree. Convert the list containing the parent ID into a tree, and the id list into a tree. we know that the database stores the tree in the form of a list (id, pid. How can we extract this tree? The simplest way is to convert the list containing the parent ID into a tree, and the id list into a tree.

We know that databases generally store trees in the form of a list (id, pid. How can we extract this tree? The simplest way is to look up the table based on the pid loop. But there is no doubt that this will produce a huge database query overhead.

Generally, the recommended method is to retrieve all the relevant data at a time, but this involves a problem: how to quickly build a tree.

I have always thought that this is a complicated operation and requires at least one Recursion. The time complexity is not O (n ).

Some time ago, a work requirement needs to be solved. I thought about it and found that this problem can be solved through a single-layer loop. the implementation is as follows:

 1 function list2Tree($listItem, $idx = 'id', $pIdx = 'pid', $childKey= 'list'){ 2     $map = array(); 3     $pMap = array(); 4  5     foreach($listItem as $item){ 6         $id = $item[$idx]; 7         $pid = $item[$pIdx]; 8         $map[$id] = &$item; 9         unset($item);10     }       11             12     foreach($map as $id => &$item){13         $pid = $item[$pIdx];14         $item[$childKey] = array();15 16         if(! isset($map[$pid])){17             $pMap[$id] = &$item; 18         }       19         else{   20             $pItem= &$map[$pid];21             $pItem[$childKey][] = &$item;22         }   23 24         unset($item, $pItem);25     }26    27     return array_shift($pMap);28 }

Test:

1 // The PATH helps identify parent-child relationships. 2 $ json = <
 
  
Result:
  
array(4) {  ["id"]=>  int(2)  ["pid"]=>  int(1)  ["path"]=>  string(3) "/se"  ["list"]=>  array(2) {    [0]=>    array(4) {      ["id"]=>      int(3)      ["pid"]=>      int(2)      ["path"]=>      string(8) "/se/4901"      ["list"]=>      array(1) {        [0]=>        array(4) {          ["id"]=>          int(5)          ["pid"]=>          int(3)          ["path"]=>          string(13) "/se/4901/mask"          ["list"]=>          array(0) {          }        }      }    }    [1]=>    array(4) {      ["id"]=>      int(6)      ["pid"]=>      int(2)      ["path"]=>      string(8) "/se/4902"      ["list"]=>      array(1) {        [0]=>        array(4) {          ["id"]=>          int(7)          ["pid"]=>          int(6)          ["path"]=>          string(13) "/se/4902/mask"          ["list"]=>          array(0) {          }        }      }    }  }}
  
The list is successfully converted into a tree.
  

  

  
Databases we know that databases generally store trees in the form of a list (id, pid. How can we extract this tree? The simplest method is according...