How to find the first common node of two linked lists in PHP (code example)

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Input two linked lists and find their first common node

1. If two singly linked lists have common nodes, then the tails must be common

2. Find the length of linked list 1, find the length of linked list 2, subtract the short linked list from the long linked list to get an n value

3. long The linked list first moves n steps, and then the two linked lists move simultaneously

4. The intersection point of the two linked lists is the first common node
   

list1 list2
len1 len2

if len1 > len2
    n=len1-len2
    for i=0;i<n;i++
        list1=list1->next
else
    n=len2-len1
    for i=0;i<n;i++
        list2=list2->next

while list1!=null
    if list1==list2 
        return list1
    list1=list1->next
    list2=list2->next
return null

<?php
class Node{
        public $data;
        public $next;
        public function __construct($data=""){
                $this->data=$data;
        }   
}
//构造一个链表
$linkList1=new Node();
$linkList1->next=null;
$temp=$linkList1;

$node1=new Node(1);
$temp->next=$node1;
$temp=$node1;

$node2=new Node(2);
$temp->next=$node2;
$temp=$node2;

$node3=new Node(3);
$temp->next=$node3;
$temp=$node3;

$node4=new Node(4);
$temp->next=$node4;
$temp=$node4;

$node5=new Node(5);
$temp->next=$node5;
$node5->next=null;

//构造一个和上面有公共结点的链表
$linkList2=new Node();
$linkList2->next=null;
$temp=$linkList2;

$node7=new Node(7);
$temp->next=$node7;
$node7->next=$node4;//链向上面链表的第四个结点


var_dump($linkList1);
var_dump($linkList2);
$commonNode=FindFirstCommonNode($linkList1,$linkList2);
var_dump($commonNode);
//找第一个公共结点
function FindFirstCommonNode($pHead1, $pHead2){
        //链表1的长度
        $len1=0;
        $temp=$pHead1->next;
        while($temp!=null){
                $temp=$temp->next;
                $len1++;
        }
        //链表2的长度
        $len2=0;
        $temp=$pHead2->next;
        while($temp!=null){
                $temp=$temp->next;
                $len2++;
        }
        $list1=$pHead1->next;
        $list2=$pHead2->next;
        //长的链表先走n步
        if($len1 > $len2){
                $n=$len1-$len2;
                for($i=0;$i<$n;$i++){
                        $list1=$list1->next;
                }
        }else{
                $n=$len2-$len1;
                for($i=0;$i<$n;$i++){
                        $list2=$list2->next;
                }

        }
        //两个链表长度一致，同时走，第一个相同的点就是第一个公共结点
        while($list1!=null){
                if($list1==$list2){
                        return $list1;
                }
                $list1=$list1->next;
                $list2=$list2->next;
        }
        return null;
}

object(Node)#1 (2) {
  ["data"]=>
  string(0) ""
  ["next"]=>
  object(Node)#2 (2) {
    ["data"]=>
    int(1)
    ["next"]=>
    object(Node)#3 (2) {
      ["data"]=>
      int(2)
      ["next"]=>
      object(Node)#4 (2) {
        ["data"]=>
        int(3)
        ["next"]=>
        object(Node)#5 (2) {
          ["data"]=>
          int(4)
          ["next"]=>
          object(Node)#6 (2) {
            ["data"]=>
            int(5)
            ["next"]=>
            NULL
          }
        }
      }
    }
  }
}
object(Node)#7 (2) {
  ["data"]=>
  string(0) ""
  ["next"]=>
  object(Node)#8 (2) {
    ["data"]=>
    int(7)
    ["next"]=>
    object(Node)#5 (2) {
      ["data"]=>
      int(4)
      ["next"]=>
      object(Node)#6 (2) {
        ["data"]=>
        int(5)
        ["next"]=>
        NULL
      }
    }
  }
}
object(Node)#5 (2) {
  ["data"]=>
  int(4)
  ["next"]=>
  object(Node)#6 (2) {
    ["data"]=>
    int(5)
    ["next"]=>
    NULL
  }
}

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