snobol, 137 bytes

Result

AC

Duration

838ms

Code [DL]

	T = INPUT
F	OUTPUT = T
	DIFFER(DUPL(' ',SIZE(T)),K = INPUT)	:S(S)F(F)
S	EQ(SIZE(K = TRIM(K) 1),SIZE(TRIM(T)))	:S(T)F(S)
T	OUTPUT = K
END

stdin

                   T                              
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
      K                                           

stdout

                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
                   T                              
      K1111111111111

stderr

strace