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'll tell you whether the number is higher or lower.

You call a pre-defined APIguess(int num)which returns 3 possible results (-1,1, or0):

-1 : My number is lower
 1 : My number is higher
 0 : Congrats! You got it!

Example:

n = 10, I pick 6.
Return 6.

二分找答案

public int guessNumber(int n) {
    if (n < 1) {
        return Integer.MIN_VALUE;
    }

    int start = 1;
    int end = n;
    while (start + 1 < end) {
        int mid = start + (end - start) / 2;
        if (guess(mid) == 0) {
            return mid;
        } else if (guess(mid) > 0) {
            start = mid;
        } else {
            end = mid;
        }
    }

    if (guess(start) == 0) {
        return start;
    } else {
        return end;
    }
}