ocaml, 129 bytes

Result

AC

Duration

785ms

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                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
           x                                      
xxxxxxxxxxxx                                      

stderr

Exception: End_of_file.

strace