[LeetCode] Guess Number Higher or Lower

We are playing the Guess Game. The game is as follows:

I pick a number from 1 to n. You have to guess which number I picked.

Every time you guess wrong, I’ll tell you whether the number is higher or lower.

You call a pre-defined API guess(int num) which returns 3 possible results (-1, 1, or 0):

-1 : My number is lower
1 : My number is higher
0 : Congrats! You got it!

Example:

n = 10, I pick 6. Return 6.

// Forward declaration of guess API.
//@param num, your guess
//@return -1 if my number is lower,1 if my number is higher,otherwise return 0
int guess(int num);
class Solution {
public:
    int guessNumber(int n) {
        //if(guess(n) == 0) return n;
        int flag;
        int b = 1,e = n;
        int guessN;
        while(b < e){
            guessN = b+(e-b)/2;
            //(b+e)/2就是错的，是因为两个大数相加可能和超过最大范围了或比较费时？？
            flag = guess(guessN);
            if(flag == 0)
                return guessN;
            if(flag == 1)
                b = guessN +1 ;
            else
                e = guessN - 1;

        }
        return b;
    }
};