题解 | #链表中环的入口结点# 快慢指针

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) {
        ListNode slow = pHead;
        ListNode fast = pHead;
        while (slow != null && fast != null) {
            slow = slow.next;
            fast = fast.next;
            if (fast == null) {
                return null;
            }
            fast = fast.next;
            
            // 有环
            if (slow == fast) {
                fast = pHead;
                while (slow != fast) {
                    slow = slow.next;
                    fast = fast.next;
                }
                return slow;
            }
        }
        return null;
    }
}