/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *entryNodeOfLoop(ListNode *head) {
        if (!head || !head -> next) return 0;
        ListNode *first = head, *second = head;

        while (first && second)
        {
            first = first -> next;
            second = second -> next;
            if (second) second = second -> next;
            else return 0;

            if (first == second)
            {
                first = head;
                while (first != second)
                {
                    first = first -> next;
                    second = second -> next;
                }
                return first;
            }
        }
        return 0;
    }
};