374. Guess Number Higher or Lower

Description

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

Binary search

最基本的二分写法。

/* The guess API is defined in the parent class GuessGame.
   @param num, your guess
   @return -1 if my number is lower, 1 if my number is higher, otherwise return 0
      int guess(int num); */

public class Solution extends GuessGame {
    public int guessNumber(int n) {
        int left = 1;
        int right = n;
        
        while (left <= right) {
            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;
            }
        }
        
        return -1;
    }
}