Linked_List_cycle2.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        if(head == NULL || head->next == NULL)
            return NULL;
        
        ListNode *slow = head;
        ListNode *fast = head;
        ListNode *entry = head;
        
        while(fast->next && fast->next->next){
            slow = slow->next;
            fast = fast->next->next;
            if(slow == fast){           //there is a cycle
                while(slow != entry){   //found the entry location
                    slow = slow ->next;
                    entry = entry->next;
                }
                return entry;
            }
        }
        return NULL;
        
    }
};