If I use one's complement to represent number, what result should I get?
For example, number 01110 ( a positive number),its one's complement should be 01110 or 10001?
if the number is 10001 ( a negative number), the one's complement value is 01110?
In your case, it would be 10001. Ones' complement means reversing all the bits in the number.
This also implies that it turns a positive number to a negative one, and vice versa. Also note that this gives some strange behavior, for example zero's complement isn't zero, but negative zero: 00000 becomes 11111. This makes arithmetic with 1's complement a bit tricky, and it is one of the reasons that computers today use 2's complement for negation.