Facebook Hacker Cup Studious Student

sstudent.lisp

(defparameter *lines* (with-open-file (in "input.txt")
		        (loop for line = (read-line in nil nil)
			   while line
			   collect line into lines
			   finally (return lines))))

(defun read-words (str)
  (cdr (read-from-string
	(format nil "(~a)" str))))

(defun cat (w1 w2)
  (format nil "~a~a" w1 w2))

(defun word< (w1 w2)
  (string< (cat w1 w2) (cat w2 w1)))

(defun build-string (lst)
  (format nil "~{~(~a~)~}" (sort lst 'word<)))

(with-open-file (out "output.txt"
		     :direction :output
		     :if-exists :supersede)
  (with-standard-io-syntax
    (format out "~{~a~%~}" (mapcar #'build-string
				   (cdr (mapcar #'read-words
						*lines*))))))

What language it is?

It's Common Lisp. :)