/**
 * 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) {
        ListNode *dummy = new ListNode; 
        dummy->val = 0;  
        dummy->next = head;
        ListNode *fast , *slow;
        fast = dummy;
        slow = dummy;
        while(fast != NULL && fast->next != NULL)
        {
            slow = slow->next;
            fast = fast->next->next;
            if(slow == fast)
            {
                ListNode *indnx1 = fast;
                ListNode *indnx2 = dummy;
                while(indnx1 != indnx2)
                {
                    indnx1 = indnx1->next;
                    indnx2 = indnx2->next;
                }
                return indnx2;
            }
        }
        return NULL;
    }
};

/**
 * 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) {
        ListNode *right = head , *left = head;
        while(right != nullptr && right->next != nullptr)
        {
            right = right->next->next;
            left = left->next;
            if(right == left)
            {
                ListNode *indnx1 = head;
                ListNode *indnx2 = right;
                while(1)
                {   
                    if(indnx1 == indnx2) return indnx1;
                    indnx1 = indnx1->next;
                    indnx2 = indnx2->next;
                }
            }
        }
        return nullptr;
    }
};