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; 
        ListNode slow = pHead, fast = pHead;
        do {
            if (fast == null || fast.next == null) return null;
            slow = slow.next;
            fast = fast.next.next;
        } while (slow != fast);
        slow = pHead;
        while (slow != fast) {
            slow = slow.next;
            fast = fast.next;
        }
        return slow;
    }
}