面试题 02.08. 环路检测

快慢指针

public class Solution {
    public ListNode detectCycle(ListNode head) {
        if (head == null){
            return head;
        }
        ListNode fast = head;
        ListNode slow = head;
        while (fast.next != null && slow.next != null && fast.next.next!=null){
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow){
                fast = head;
                while (slow != fast){
                    slow = slow.next;
                    fast = fast.next;
                }
                return slow;
            }
        }
        return null;
    }
}