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.

Solution

// 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 l = 1, r = n, m;
        if ( guess(l) == 0 ) return l;
        if ( guess(r) == 0 ) return r;
        while ( l != r ) {
            m = l/2 + r/2 + (l%2 + r%2)/2;
            cout << m << endl;
            if ( guess(m) == 0 ) return m;
            if ( guess(m) == 1 ) l = m;
            if ( guess(m) == -1 ) r = m;
        }
        return l;
    }
};

Notes

• Note the trick to avoid overflow! [always remember this!]