PHP で 2 つのソートされたリンク リストをマージする方法の説明

<?php
/*class ListNode{
 var $val;
 var $next = NULL;
 function __construct($x){
  $this->val = $x;
 }
}*/
function Merge($pHead1, $pHead2)
{
 if($pHead1 == NULL)
  return $pHead2;
 if($pHead2 == NULL)
  return $pHead1;
 $reHead = new ListNode();
 if($pHead1->val < $pHead2->val){
  $reHead = $pHead1;
  $pHead1 = $pHead1->next;
 }else{
  $reHead = $pHead2;
  $pHead2 = $pHead2->next;
 }
 $p = $reHead;
 while($pHead1&&$pHead2){
  if($pHead1->val <= $pHead2->val){
   $p->next = $pHead1;
   $pHead1 = $pHead1->next;
   $p = $p->next;
  }
  else{
   $p->next = $pHead2;
   $pHead2 = $pHead2->next;
   $p = $p->next;
  }
 }
 if($pHead1 != NULL){
  $p->next = $pHead1;
 }
 if($pHead2 != NULL)
  $p->next = $pHead2;
 return $reHead;
}