142

/**
 * 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 *slow, *fast;
        slow = head;
        fast = head;
        bool cyclePresent = false;
        while(fast && fast->next){
            slow = slow->next;
            fast = fast->next->next;
            if(slow == fast){
                cyclePresent = true;
                break;
            }
        }
        if(!cyclePresent) return NULL;
        slow = head;
        while(slow != fast){
            slow = slow->next;
            fast = fast->next;
        }
        return slow;
    }
};