Solve the idea of tree-like data (adjacency list) with List

Using list to process tree data (adjacency list)
An existing array

$d = Array (
  Array (' Announcements ', 1, 0),
  Array (' article ', 2, 0),
  Array (' Article 1 ', 3, 2),
  Array (' Article 2 ', 4, 2),
  Array (' Article 1 comments ', 5, 3),
  Array (' Article 2 Comments ', 6, 4),
  Array (' Article 1 reviews 1 ', 7, 3),
  Array (' Article 1 comment comments ', 8, 5),
);

Expect the following output

Notice
Article
  Article 1
    Article 1 comments
      Article 1 Comment comments
    Posts 1 Comments 1
  Article 2
    Article 2 Comments

So you can

foreach ($d as $t) list ($a [$pid] [$id], $id, $pid) = $t;

Get

Array
(
    [0] = = Array
        (
            [1] = = Announcement
            [2] = = Articles
        )

    [2] = = Array
        (
            [3] = = Article 1
            [4] = = Article 2
        )

    [3] = = Array
        (
            [5] = + Article 1 comments
            [7] = + Article 1 comments 1
        )

    [4] = = Array
        (
            [6] = + Article 2 comments
        )

    [5] = = Array
        (
            [8] = + Article 1 Comment Comment
        )

)

As you can see, the data is clustered in column 3rd.
Then a recursive function can be used to achieve the presentation of the data

function foo ($ar, $pid =0, $deep =0) {
  foreach ($ar [$pid] as $k = = $v) {
    printf ("%s%s\n", Str_repeat (", $deep), $v);
    if (Isset ($ar [$k]) foo ($ar, $k, $deep +2);
  }
}

Call Foo ($a);

------Solution--------------------
Moderators are a great guy.
------Solution--------------------

The owner of the infinite-level tree has a unique feeling.
Every time I see a new harvest.
------Solution--------------------

------Solution--------------------
Front Row Study!
------Solution--------------------
Study. Oh

------Solution--------------------

------Solution--------------------
That's how it turns out.
------Solution--------------------
Really concise, study.
------Solution--------------------
Good writing, study
------Solution--------------------

 static void Main (string[] args) 
 {
 Double A, B, C, p, h, area; 
 Console.Write ("Please enter the edge of the Triangle A:"); 
 String s = Console.ReadLine (); 
 A = double. Parse (s); 
 Console.Write ("Please enter the edge of the Triangle B:"); 
 S = console.readline (); 
 B = Double. Parse (s); 
 Console.Write ("Please enter the edge of the Triangle C:"); 
 S = console.readline (); 
 C = Double. Parse (s); 
 if (a > 0 && B > 0 && C > 0 && a + B > C && a + C > B &                & B + C > a) 
 {
 Console.WriteLine (three sides of the triangle are: a={0},b={1},c={2} ", A, B, c); 
 p = A + B + C; 
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