your code is giving a segmentation fault.

for this input

5
pue
heubsbn
feazhaxpux
hmhcy
tmp

Try doing this in your local machine.

gcc -pipe -O1 -std=c99 code.c 
gdb a.out 
run < input01.txt

it gives the error

Program received signal SIGSEGV, Segmentation fault.
0x00000000004007db in change (z=112 'p', z@entry=<error reading variable: DWARF-2 expression error: Loop detected (257).>, c=97 'a', i=2, 
i@entry=<error reading variable: DWARF-2 expression error: Loop detected (257).>) at soln.c:45
45    change(z,c,i);