ocaml, 136 bytes

Result

AC

Duration

788ms

Code [DL]

let rec f x=let c=input_char stdin in
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

                                        T         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                                        x         
                      xxxxxxxxxxxxxxxxxxx         

stderr

Exception: End_of_file.

strace