剑指offer:55.链表中环的入口结点

题目描述
给一个链表，若其中包含环，请找出该链表的环的入口结点，否则，输出null。

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

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

    public ListNode EntryNodeOfLoop(ListNode pHead)
    {
        if(pHead==null||pHead.next==null)return null;
        ListNode fast=pHead,slow=pHead;
        //快慢指针法判断是否有环
        do{
            fast=fast.next.next;
            slow=slow.next;
        }while(fast!=slow);
        //将快指针置为链表头，慢指针为相遇处，
        //快慢指针同样速度前进，再次相遇时为环入口
        fast=pHead;
        while(fast!=slow){
            fast=fast.next;
            slow=slow.next;
        }
        return slow;
    }
}

//更准确的写法
/*
 public class ListNode {
    int val;
    ListNode next = null;

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

    public ListNode EntryNodeOfLoop(ListNode pHead)
    {
        if(pHead==null)return null;
        ListNode fast=pHead,slow=pHead;
        while(fast!=null && fast.next!=null){
            fast=fast.next.next;
            slow=slow.next;
            if(fast==slow){
                ListNode slow2=pHead;
            while(slow2!=slow){
                slow2=slow2.next;
                slow=slow.next;
            }
            return slow;
            }
        }
        return null;
    }
}