牛客网--关于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。

代码

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

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

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