Linked List Cycle II

1. Title

Linked List Cycle II

2. Http address

https://leetcode.com/problems/linked-list-cycle-ii/

3. The question

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Note: Do not modify the linked list.

Follow up:
Can you solve it without using extra space?

 4. My code(AC)

•  // Accepted
         public static ListNode detectCycle(ListNode head) {
             
             if( head == null)
                 return null;
             ListNode last ,pre;
             last = head;
             pre = head.next;
             
             if( pre == last)
                 return head;
             while( pre != null)
             {
                 if( pre == last)
                 {
                     break;
                 }
                 last = last.next;
                 pre = pre.next;
                 if( pre != null)
                 {
                     pre = pre.next;
                 }else{
                     pre = null;
                 }
             }
             
             if ( pre == null)
             {
                 return null;
             }
             ListNode second = pre.next;
             ListNode first = head;
             pre.next = null;
             
             int lenA,lenB;
            
             ListNode p = first;
             lenA = 0 ;
             while( p != null)
             {
                 lenA++;
                 p = p.next;
             }
             lenB = 0 ;
             p = second;
             while( p != null)
             {
                 lenB++;
                 p = p.next;
             }
             
             if( lenA >= lenB)
             {
                 int dis = lenA - lenB;
                 while( dis > 0 && first != null)
                 {
                     first = first.next;
                     dis--;
                 }
                 
             }else{
                 int dis = lenB - lenA;
                 while( dis > 0 && second != null)
                 {
                     second = second.next;
                     dis--;
                 }
             }
             
             while( first != null && second != null && first != second)
             {
                 first = first.next;
                 second = second.next;
             }
             return second;
             
           
          }