[LeetCode] Linked List Cycle

题目：

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

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

解答：

/** * 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;                if(head == NULL || head -> next == NULL || head -> next -> next == NULL) {            return false;        }        else {            while(fast != NULL && fast -> next != NULL) {                fast = fast -> next -> next;                slow = slow -> next;                            if(fast == slow) {                    return true;                }            }            return false;        }    }};

经典算法：快慢指针判断链表是否有环，快指针每次前进两步，慢指针每次前进一步，如果有环，则快慢指针必定在某一时刻相遇；如果没有环，则快指针先到达链表结尾。