题解 | #链表中环的入口结点#

首先先写出判断有没有环，没有返回null；

/*
 public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}
*/
public class Solution {

    public ListNode EntryNodeOfLoop(ListNode pHead) {
        ListNode slow,fast;
        fast = slow = pHead;
        while(fast != null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;
            if(fast == slow){有环的情况，代码待补充}
        }
           return null;
    }
}

然后处理有环的情况返回环的起点；

/*
 public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}
*/
public class Solution {

    public ListNode EntryNodeOfLoop(ListNode pHead) {
        ListNode slow,fast;
        fast = slow = pHead;
        while(fast != null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;
            if(fast == slow){ 
            //有环的情况，返回起点
            slow = pHead;
            while(slow != fast){
            fast = fast.next;
            slow = slow.next;
            }
            return slow;
            }
          //结束
        }
           return null;
    }
}