Let there is a string 'S'.
Now, we want to change this string by some commands.
There are 3 type of command; strncpy, strncat, strncmp.
strncpy : if the integer 'n' and string 'str' is inputed,
change first 'n' strings of 'S' to first 'n' strings of 'str'.
for example, if 'S' is "ABCDE", 'str' is "abcde", n is 3, S become "abcDE"
strncat : if the integer 'n' and string 'str' is inputed,
concat the first 'n' strings of 'str'. after string 'S'
for example, if 'S' is "ABCDE", 'str' is "abcde", n is 3, S become "ABCDEabc"
strncmp : compare the first 'n' string between 'S' and 'str'
if 'S' is smaller than 'str' (ASCII-order), print -1;
else if 'S' is bigger than 'str', print 1;
if two are same, print 0
for example, 'S' is "ABCDE" and 'str' is "abcdefg" and n is 3,
result is -1,
if 'str' is "ABCAAAAA" and n is 3.
result is 0
if n is 4, result is 1.
