2.rkt

#lang racket
(define str
;; put large string from src code here.
#<<HERE
...
HERE
)

;; h = [Hash Char (Pair Nat Nat)]
;; where (Pair Nat Nat) represents occurrence count and last occurrence position.
(define-values (h _)
  (for/fold [(h (hash)) (i 0)]
    [(c (in-string str))]
    (values
     (hash-update h c (λ (p) (cons (add1 (car p)) i)) (cons 0 i))
     (add1 i))))

(apply string
       (map car (sort (for/list ([p (in-hash-pairs h)]
                                 #:when (< (cadr p) 10))
                        p)
                      < #:key cddr)))