【LeetCode】141. Linked List Cycle (2 solutions)

Linked List Cycle

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

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

解法一：

使用unordered_map记录当前节点是否被访问过，如访问过说明有环，如到达尾部说明无环。

/*** 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) {unordered_map<ListNode*, bool> visited;while(head != NULL){if(visited[head] == true)return true;visited[head] = true;head = head->next;}return false;}
};

解法二：不使用额外空间

设置快慢指针，

fast每次前进两步，slow每次前进一步，如相遇说明有环，如到达尾部说明无环。

/*** 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;do{if(fast != NULL)fast = fast->next;elsereturn false;if(fast != NULL)fast = fast->next;elsereturn false;slow = slow->next;}while(fast != slow);return true;}
};

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