374.GuessNumber.cs

// 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 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.
// Example 1:
// Input: n = 10, pick = 6
// Output: 6
// Example 2:
// Input: n = 1, pick = 1
// Output: 1
// Example 3:
// Input: n = 2, pick = 1
// Output: 1

/** 
 * 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);
 */

public class Solution : GuessGame {
    public int GuessNumber(int n) {
        int low = 1;
        int high = n;
        
        while(low <= high)
        {
            int mid = low +(high - low) /2; 
            int res = guess(mid);
            if(res == 0)
            {
                return mid;
            }
            else if (res == -1 )  
            {
                high = mid -1;
            }
            else if(res == 1) 
            {
               low = mid +1; 
            }
        }
        return -1;
    }
}