Guessing a number knowing only if the number proposed is lower or higher?

Question

I need to guess a number. I can only see if the number I\'m proposing is lower or higher. Performance matters a whole lot, so I thought of the following algorithm:

Le

Answer 1

int a = 1, b = n+1, guess = average of previous answers;

while(guess is wrong) {

    if(guess lower than answer) {a = guess;}
    else if(guess higher than answer) {b = guess;}

    guess = (a+b)/2;

} //Go back to while

You got to add +1 to n else you can never get n since it's an int.