swift, 119 bytes

Result

AC

Duration

1073ms

Code [DL]

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

stdin

                                   T              
                                                  
                                                  
          K                                       

stdout

                                   x              
                                   x              
                                   x              
          xxxxxxxxxxxxxxxxxxxxxxxxxx              

stderr

strace