374. 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.

需要注意的是，上面的lower是正确的数字相对于猜的数字。

// 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) {

        int lb = 1, ub = n;
        int myGuess;
        while(lb <= ub){
            int mid = lb + (ub - lb)/2;
            if( (myGuess = guess(mid)) == 0)
                return mid ;
            else if(myGuess == 1)
                lb = mid + 1;
            else
                ub = mid - 1;
        }
        return -1;
    }
};