LeetCode 374. Guess Number Higher or Lower

原题链接在这里：https://leetcode.com/problems/guess-number-higher-or-lower/

题目：

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.

题解：

Binary Search, guess mid为0则return mid. guess mid为-1, 就在左半面继续猜. guess mid 为1, 就在右半面继续猜.

Time Complexity: O(logn). Space: O(1).

AC Java:

/* 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 l = 1;
        int r = n;
        while(l <= r){
            int mid = l + (r-l)/2;
            int result = guess(mid);
            if(result < 0){
                r = mid-1;
            }else if(result > 0){
                l = mid+1;
            }else{
                return mid;
            }
        }
        return -1;
    }
}

类似First Bad Version.