Odd Occurrences In Array

Lisp Odd Occurrences In Array

(defun odd-occurrences-in-array (a)
  (let ((counts (make-hash-table :test 'eql)))
    (dolist (v a)
      (incf (gethash v counts 0)))
    (loop for v being the hash-keys of counts using (hash-value c)
          when (oddp c)
            return v)))

This uses XOR to cancel out pairs, leaving only the value that appears an odd number of times.