text: /**
   * Definition for singly-linked list.
   * struct ListNode {
   *     int val;
   *     ListNode *next;
   *     ListNode(int x) : val(x), next(NULL) {}
   * };
   
   */
  class Solution {
  public:
      ListNode* middleNode(ListNode* head) {
          
          ListNode *slow = head, *fast = head;
          
          while (fast && fast->next){
              slow = slow -> next;
              fast = fast -> next ->next;
          }
          
          return slow;
      }
  };
  