Guess Number Higher or Lower.java

E
1516346617
tags: Binary Search

binary search 公式

```
/*
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.
*/

/* 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); */

/*
Thoughts:
Like binary search, based on result, guess another one.
*/

/* 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) {
        if (n <= 0) {
            return n;
        }
        int start = 0;
        int end = n;
        while (start + 1 < end) {
            int mid = start + (end - start) / 2;
            int result = guess(mid);
            if (result == 0) {
                return mid;
            } else if (result == 1) {
                start = mid;
            } else {
                end = mid;
            }
        }
        return guess(start) == 0 ? start : end;
    }
}
```