leetcode - Linked List Cycle

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

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

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */struct ListNode{int val;ListNode *next;ListNode(int x) : val(x), next(NULL) {}};/*三種情況：1.head == null,返回false2.List只有一個結點，返回false3.這種情況，要用兩個指標，fast,slow.分別遍曆list，因為，fast比slow快兩倍，所以，找到list的cycle應該是可行的。當fast == slow的時候就可以判斷存在list的cycle,並且返回true,如果，遍曆結束，也沒有找到，那麼就返回false.*/class Solution {public:    bool hasCycle(ListNode *head) {if(head == NULL) return false;if(head->next == NULL) return true;ListNode *fast = head->next;ListNode *slow = head;while(fast != NULL && fast->next != NULL){if(fast == slow || fast->next == slow){return true;}slow = slow->next;fast = fast->next->next;}return false;    }};

leetcode - Linked List Cycle