链表是否有环，环中有多少节点，环的入口节点

  1 //public class ListNode {
  2 //    int val;
  3 //    ListNode next;
  4 //}
  5 
  6 public class ListNodeMeet {
  7     
  8     public static ListNode enterNode(ListNode head){
  9         if(nodesInCycle(head) == 0){
 10             return null;
 11         }
 12         else{
 13             int nums = nodesInCycle(head);
 14             ListNode slow = head;
 15             ListNode fast = head;
 16             for(int i = 0; i < nums; i++){
 17                 fast = fast.next;
 18             }
 19             while(slow != fast){
 20                 slow = slow.next;
 21                 fast = fast.next;
 22             }
 23             return slow;
 24         }
 25     }
 26     
 27     public static int nodesInCycle(ListNode head){
 28         if(!hasCycle(head)){
 29             return 0;
 30         }
 31         else{
 32             ListNode slow = head;
 33             ListNode fast = head;
 34             while(true){
 35                 if(slow.next == fast.next.next){
 36                     ListNode meet = slow.next;
 37                     ListNode cur = slow.next.next;
 38                     int count = 1;
 39                     while(cur != meet){
 40                         count++;
 41                         cur = cur.next;
 42                     }
 43                     return count;
 44                 }
 45                 slow = slow.next;
 46                 fast = fast.next.next;
 47             }
 48         }
 49     }
 50     
 51     public static boolean hasCycle(ListNode head){
 52         
 53         if(head == null){
 54             return false;
 55         }
 56         
 57         ListNode slow = head;
 58         ListNode fast = head;
 59         
 60         while(true){
 61             if(slow.next == null || fast.next.next == null){
 62                 return false;
 63             }
 64             if(slow.next == fast.next.next){
 65                 return true;
 66             }
 67             slow = slow.next;
 68             fast = fast.next.next;
 69         }
 70     }
 71     
 72     public static void main(String[] args) {
 73         ListNode node1 = new ListNode();
 74         ListNode node2 = new ListNode();
 75         ListNode node3 = new ListNode();
 76         ListNode node4 = new ListNode();
 77         ListNode node5 = new ListNode();
 78         ListNode node6 = new ListNode();
 79         
 80         node1.val = 1;
 81         node2.val = 2;
 82         node3.val = 3;
 83         node4.val = 4;
 84         node5.val = 5;
 85         node6.val = 6;
 86         
 87         node1.next = node2;
 88         node2.next = node3;
 89         node3.next = node4;
 90         node4.next = node5;
 91         node5.next = node6;
 92         node6.next = node3;
 93         //node6.next = null;
 94         
 95         System.out.println("是否有环: "+hasCycle(node1));
 96         System.out.println("环中节点个数: "+nodesInCycle(node1));
 97         System.out.println("环的入口节点: "+enterNode(node1));
 98 
 99     }
100 
101 }