Language: C C++ D Haskell Lua OCaml PHP Perl Plain Text Python Ruby Scheme Tcl (define (sudoku puzzle) (define (safe? filled digit cell) (cond ((null? filled) #t) ((and (= (vector-ref (car filled) 0) (vector-ref cell 0)) (char=? (vector-ref (car filled) 3) digit)) #f) ((and (= (vector-ref (car filled) 1) (vector-ref cell 1)) (char=? (vector-ref (car filled) 3) digit)) #f) ((and (= (vector-ref (car filled) 2) (vector-ref cell 2)) (char=? (vector-ref (car filled) 3) digit)) #f) (else (safe? (cdr filled) digit cell)))) (define (next digit) (integer->char (+ (char->integer digit) 1))) (define (new old digit) (vector (vector-ref old 0) (vector-ref old 1) (vector-ref old 2) digit)) (let scan ((s 0) (empty '()) (filled '())) (if (< s 81) (let* ((row (quotient s 9)) (col (modulo s 9)) (box (+ (* (quotient row 3) 3) (quotient col 3))) (digit (string-ref puzzle s)) (cell (vector row col box digit))) (if (char=? digit #\0) (scan (+ s 1) (cons cell empty) filled) (scan (+ s 1) empty (cons cell filled)))) (let solve ((empty empty) (filled filled)) (if (pair? empty) (let try ((cell (car empty)) (digit (next (vector-ref (car empty) 3)))) (cond ((char<? #\9 digit) ; backtrack (solve (cons (car filled) (cons (new cell #\0) (cdr empty))) (cdr filled))) ((safe? filled digit cell) ; advance (solve (cdr empty) (cons (new cell digit) filled))) (else (try cell (next digit))))) ; try next digit (let ((str (make-string 81 #\0))) (do ((filled filled (cdr filled))) ((null? filled) str) (let* ((cell (car filled)) (s (+ (* (vector-ref cell 0) 9) (vector-ref cell 1)))) (string-set! str s (vector-ref cell 3)))))))))) (display (sudoku "700100000020000015000006390200018000040090070000750003078500000560000040000001002")) Private [?] Run code