Bob plays the "guess the number right" with Alice recently,the game's rule is that Alice give Bob a upper limit number N ,then he write any of a number on paper which Bob can't know what it is and the number must be between 1 and N.Bob has many chances to guess the number and every time when Bob guesses Alice will tell him if his is bigger than the correct number or small than the correct number until he is right.
Now the Bob wanted to use binary search method to guess the right number, because he knows this method is quite fast to find the right number.