snobol, 128 bytes

Result

AC

Duration

734ms

Code [DL]

	T = INPUT
F	OUTPUT = T
	DIFFER(DUPL(' ',SIZE(T)),K = INPUT)	:F(F)
S	EQ(SIZE(K = TRIM(K) 1),SIZE(TRIM(T)))	:F(S)
	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                           
K1111111111111111111111

stderr

strace