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;
        ListNode fast = pHead;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast) {
                break;
            }
        }
        if (fast == null || fast.next == null ) {
            return null;
        }
        ListNode ptr = pHead;
        ListNode ptr1 = slow;
        while (ptr != ptr1) {
            ptr1 = ptr1.next;
            ptr = ptr.next;
        }
        System.out.print(ptr);
        return ptr;
    }
}