链表//环形链表

给定一个链表，判断链表中是否有环。

进阶：
你能否不使用额外空间解决此题？

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode fast;
        ListNode slow;
        if(head == null||head.next == null)
            return false;
        else{
            fast = head.next;
            slow = head;
            while(fast != null){
                if(fast == slow)
                    return true;
                else{
                    fast = fast.next;
                    if(fast == null)
                        return false;
                    else{
                        fast = fast.next;
                        slow = slow.next;
                    }
                }
            }
        }
        return false;
    }
}

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        ListNode *fast;
        ListNode *slow;
        
        if(head == nullptr||head->next == nullptr)
            return false;
        else{
            fast = head->next;
            slow = head;
            while(fast != nullptr){
                if(fast == slow)
                    return true;
                else{
                    fast = fast->next;
                    if(fast == nullptr)
                        return false;
                    else{
                        fast = fast->next;
                        slow = slow->next;
                    }
                }
            }
        }
        return false;
    }
};