题意：

    Given a linked list, determine if it has a cycle in it.

    Follow up:
    Can you solve it without using extra space?

     思路：

     如何判断一个链表存在环？可以设置两个指针，一个快，一个慢，沿着链表走，在无环的情况下，快指针很快到达终点。如果存在环，那么，快指针就会陷入绕圈，而最后被慢指针追上。

     代码：

     C++:

/**
 * 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) {
        /*初始都指向 head */
        ListNode *pfast = head,*pslow = head;
        do{
            /*快指针两倍速*/
            if(pfast != NULL) pfast = pfast->next;
            if(pfast != NULL) pfast = pfast->next;

            /*到达 null，无环 */
            if(pfast == NULL) return false;

            /*慢指针*/
            pslow = pslow->next;
        }while(pfast != pslow);
        return true;
    }
};

     Python:

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    # @param head, a ListNode
    # @return a boolean
    def hasCycle(self, head):
        
        pfast = head
        pslow = head
        
        while True:
            if pfast != None:
                pfast = pfast.next
            if pfast != None:
                pfast = pfast.next
            
            if pfast == None:
                return False
            
            pslow = pslow.next
            
            if pfast == pslow:
                break
        
        return True
        

【LeetCode】Linked List Cycle