力扣 142.环形链表Ⅱ【详细解释】

一、题目

二、思路

三、代码

/*** Definition for singly-linked list.* class ListNode {*     int val;*     ListNode next;*     ListNode(int x) {*         val = x;*         next = null;*     }* }*/
public class Solution {public ListNode detectCycle(ListNode head) {ListNode fast = head, slow = head;// 判断是否有环while (fast != null && fast.next != null) {slow = slow.next;fast = fast.next.next;// slow 和 fast 相遇即表示有环存在if (slow == fast) {ListNode index1 = head;ListNode index2 = slow;// 有环则一定有入口点while (index1 != index2) {index1 = index1.next;index2 = index2.next;}return index1;}}return null;}
}