ocaml, 130 bytes

Result

AC

Duration

740ms

Code [DL]

let rec f?(c=input_char stdin)x=let x=(match c with 'T'->51|'K'->x-50|_->x+1)mod 51 in
print_char(if x<1 then 'x' else c);f x;;f 0

stdin

                               T                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
  K                                               

stdout

                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
                               x                  
  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx                  

stderr

Exception: End_of_file.

strace