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;
else
return false;
if(fast != NULL)
fast = fast->next;
else
return false;
slow = slow->next;
}while(fast != slow);
return true;
}
};
