swift, 117 bytes

Result

AC

Duration

989ms

Code [DL]

var x=0
while let l=readLine(){for i in l{x=i=="T" ?0:i=="K" ?x-49:(x+1)%50;print(x<1 ?"x":i,terminator:"")}
print()}

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         
                      xxxxxxxxxxxxxxxxxxx         

stderr

strace