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