题解 | #链表中环的入口结点#

import java.util.*;
/*
 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;
        }
	  // 使用双指针，一个快指针，一个慢指针
        ListNode fastNode = pHead;
        ListNode slowNode = pHead;
        while (fastNode != null && slowNode != null){
            fastNode = fastNode.next.next;
            slowNode = slowNode.next;
		  // 如果说两个节点一致，那么证明有环，否则就是五环
            if (fastNode == slowNode){
                ListNode temp = pHead;
			  // 一直找，直到找到头节点和环的节点一致的节点，返回的就是环的入口
                while (temp != slowNode){
                    temp = temp.next;
                    slowNode = slowNode.next;
                }
                return temp;
            }
        }
        return null;
    }
}