Lisp Missing Integer
(defun missing-integer (a)
  (let ((sorted (sort (remove-duplicates (copy-list a) :test #'=) #'<))
        (min 1))
    (loop for v in sorted
          do (when (> v 0)
               (if (/= min v)
                   (return)
                   (incf min))))
    min))

This records the positive numbers that exist, then returns the smallest positive value that is still missing.