题目：

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

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

 

Example 1:

Input: head = [3,2,0,-4], pos = 1Output: trueExplanation: There is a cycle in the linked list, where tail connects to the second node.

Example 2:

Input: head = [1,2], pos = 0Output: trueExplanation: There is a cycle in the linked list, where tail connects to the first node.

Example 3:

Input: head = [1], pos = -1Output: falseExplanation: There is no cycle in the linked list.

分析：

给定一个链表，判断链表中是否有环。

遍历链表将节点存在map中，每次添加时，都判断下，是否已经存在。

还可以用快慢指针，慢指针一次走一个，快指针一次走两个，如果链表存在环的话，快慢指针终会相等。

程序：

/** * 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) {        map
     
       m;        while(head){            if(m[head] == 1)                return true;            m[head] = 1;            head = head->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 = head;        ListNode* slow = head;        while(fast && slow){            fast = fast->next;            slow = slow->next;            if(fast){                fast = fast->next;            }            else                break;            if(fast == slow)                return true;        }        return false;    }};