(defun triangle (a)
  (let* ((sorted (sort (copy-list a) #'<))
         (vec (coerce sorted 'vector))
         (c (length vec)))
    (if (< c 3)
        0
        (progn
          (loop for i from 0 below (- c 2)
                do (when (and (> (aref vec i) 0)
                              (> (aref vec i) (- (aref vec (+ i 2)) (aref vec (1+ i)))))
                     (return-from triangle 1)))
          0))))