algorithm/hasCycle.cpp at master · leobuzhi/algorithm · GitHub

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

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
//my Solution
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if(head==nullptr)
            return false;
        ListNode* slow,*fast;
        slow=head->next;
        if(slow==nullptr)
            return false;
        else
            fast=slow->next;
        while(slow!=fast)
        {
            if(fast==nullptr)
                return false;
            slow=slow->next;
            if(fast->next==nullptr)
                return false;
            else
                fast=fast->next->next;
        }
        return true;
    }
};


/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
//better Solution
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if(head == NULL || head -> next == NULL)
            return false;

        ListNode *fast = head;
        ListNode *slow = head;

        while(fast -> next && fast -> next -> next){
            fast = fast -> next -> next;
            slow = slow -> next;
            if(fast == slow)
                return true;
        }

        return false;
    }
};