一道无限级分类题的 PHP 实现

给出如下的父子结构（你可以用你所用语言的类似结构来描述，第一列是父，第二列是子），将其梳理成类似如图的层次父子结构。
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‘)]

<?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‘]
];

// 根据父id找出子信息
function level($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;
}

// 构建顶级分类
$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;
}