[LeetCode] Linked List Cycle

 

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?

解題思路：

1、最基本的辦法是用一個set來存儲所有已經出現過的指針。若出現重復，則表示有環，若沒有重復，則沒有環。

 

/**
 * 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* p = head;
        set s;
        while(p!=NULL){
            if(s.find(p)!=s.end()){
                return true;
            }
            s.insert(p);
            p=p->next;
        }
        return false;
    }
};

2、雙指針方法。設立兩個指針，一個指針每次走一步，另外一個指針每次走兩步。若兩個指針相遇，表示有環。不相遇，則表示無環。具體見：http://blog.csdn.net/kangrydotnet/article/details/45154927

 

 

/**
 * 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* one = head;
        ListNode* two = head;
        while(two!=NULL && two->next!=NULL){
            one=one->next;
            two=two->next->next;
            if(one==two){
                return true;
            }
        }
        return false;
    }
};