递归反转整个链表 + 双指针

// 单链表节点的结构

public class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
}  

递归反转整个链表

ListNode reverse(ListNode head) {
    if (head.next == null) 
        return head;
    ListNode last = reverse(head.next);
    head.next.next = head;
    head.next = null;
    return last;
}

boolean hasCycle(ListNode head) {
    ListNode fast, slow;
    fast = slow = head;
    while (fast != null && fast.next != null) {
        fast = fast.next.next;
        slow = slow.next;
        if (fast == slow) 
            return true;
    } 
    return false;
}