(LeetCode OJ) 141. Linked List Cycle

141. Linked List Cycle

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Total Accepted: 88665 Total
Submissions: 241622 Difficulty: Medium

Given a linked list, determine if it has a cycle in it.
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Can you solve it without using extra space?

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(M) Linked List Cycle II

 

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 //思路首先：如果有環？遍歷鏈表將無法走完，如果無環終會走到尾為NULL的位置
 //讓一個指針每次走一個，一個指針每次走兩個位置，如果其中一個為NULL則無環，如果相遇了
 //則有環
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if(head==NULL)
            return false;
        ListNode *showNode=head;
        ListNode *fastNode=head;
        while(true)
        {
            if(showNode->next!=NULL)
                showNode=showNode->next;
            else
                return false;
            if(fastNode->next!=NULL && fastNode->next->next!=NULL)    
                fastNode=fastNode->next->next;
            else
                return false;    
            if(showNode==fastNode)
                return true;
        }
        return false;
    }
};