PHP implementation to find out the list of links in the link list of the entry node detailed

This paper mainly introduces the implementation of PHP to find the link list of the entry node, involving PHP for the loop chain list of the traversal, find, calculation and other related operational skills, the need for friends can refer to, hope to help everyone.

Problem

A linked list contains the ring, please find the link to the list of the entry node.

Solution Ideas

The first step is to find the loop in the meeting point. Using P1,P2 to point to the list head, p1 each step, p2 each walk two steps, until P1==P2 find the meeting point in the ring.
Step two, find the entrance to the ring. The next step, when P1==p2, p2 through the number of nodes is 2x,p1 through the number of nodes X, set the ring has n nodes, p2 than P1 walk a circle has 2x=n+x; N=x, can see P1 actually walked a ring step number, then let P2 point to the list head, P1 position unchanged, p1,p2 every step until p1==p2; At this point the P1 points to the inlet of the ring. (I don't know yet)

Implementation code

<?php/*class listnode{var $val;  var $next = NULL;  function __construct ($x) {$this->val = $x;  }}*/function Entrynodeofloop ($pHead) {if ($pHead = = NULL | | $pHead->next = = null) return null;  $p 1 = $pHead;  $p 2 = $pHead;    while ($p 2!=null && $p 2->next!=null) {$p 1 = $p 1->next;    $p 2 = $p 2->next->next;      if ($p 1 = = $p 2) {$p 2 = $pHead;        while ($p 1!= $p 2) {$p 1 = $p 1->next;      $p 2 = $p 2->next;    } if ($p 1 = = $p 2) return $p 1; }} return null;}