链表－leetcode 141 Linked List Cycle

题目链接：Linked List Cycle

题解：
/*
思路：借助一个set，将访问过的节点存储到set中，如果再次访问到set，则返回true，否则返回false;
Time Complexity:O(N)
Space Complexity:(N)
*/

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */ #include <set>class Solution {public:    bool hasCycle(ListNode *head) {        if(!head)return false;        set<ListNode *>listSet;        while(head){            if(listSet.find(head)!=listSet.end())return true;            else{                listSet.insert(head);                head=head->next;            }        }        return false;    }};

题目follow up:能否不用额外的空间完成任务，当然也是可以的
思路：借助链表的好帮手，快慢指针。

/** * 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){         /*             Time Complexity:O(N)         */         if(!head)return false;         ListNode *fast=head;         ListNode *slow=head;         while(fast->next && fast->next->next){             slow=slow->next;             fast=fast->next->next;             if(slow==fast)return true;         }         return false;     } }