text:  class Solution {
  public:
      ListNode* XXX(ListNode* head, int n) {
          if(!head | !head -> next) return NULL;
          ListNode * fast = head, *slow = head;
          for(int i = 0; i < n; i++){
              fast = fast -> next;
          }
          if(!fast){
              return head -> next;    
          }
          
          while(fast -> next){
              fast = fast -> next;
              slow = slow -> next;
          }
          slow -> next = slow -> next -> next;
          return head;
      }
  };
  
  