Guess Number Higher or Lower

Problem

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 will tell you whether the number I picked is higher or lower than your guess.

You call a pre-defined API int guess(int num), which returns three possible results:

• -1: Your guess is higher than the number I picked (i.e. num > pick).
• 1: Your guess is lower than the number I picked (i.e. num < pick).
• 0: your guess is equal to the number I picked (i.e. num == pick).

Return the number that I picked.

Constraints

• 1 <= n <= 231 - 1
• 1 <= pick <= n

Solution

The problem Guess Number Higher or Lower can be solved using a binary search.

Implementation

/**
 * Forward declaration of guess API.
 * @param  num   your guess
 * @return 	     -1 if num is higher than the picked number
 *			      1 if num is lower than the picked number
 *               otherwise return 0
 * int guess(int num);
 */

class Solution
{
  public:
    int guessNumber(int n)
    {
        int left = 1, right = n;
        while (true)
        {
            int mid = left + (right - left) / 2;
            int res = guess(mid);

            if (res < 0)
                right = mid - 1;
            else if (res > 0)
                left = mid + 1;
            else
                return mid;
        }
    }
};