public class Solution {
    public ListNode detectCycle(ListNode head) {
        if (head == null) {
            return null;
        }
        ListNode slow = head, fast = head;//快指针每次走两步，慢指针每次走一步
        while (fast != null) {
            slow = slow.next;//走一步
            if (fast.next != null) {
                fast = fast.next.next;//走两步
            } else {
                return null;
            }
            if (fast == slow) {//快指针和慢指针相遇时
                ListNode ptr = head;//从头指针出发一个指针，直到与慢指针相遇，这时候两个指针的相遇点就是入环点
                while (ptr != slow) {
                    ptr = ptr.next;
                    slow = slow.next;
                }
                return ptr;
            }
        }
        return null;
    }
}

作者：LeetCode-Solution
链接：https://leetcode-cn.com/problems/linked-list-cycle-ii/solution/huan-xing-lian-biao-ii-by-leetcode-solution/
来源：力扣（LeetCode）
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