374 Guess Number Higher or Lower

1. Question

We are playing the Guess Game. The game is as follows:
I pick a number from1ton. 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
-1 : My number is lower
2
1 : My number is higher
3
0 : Congrats! You got it!
Copied!
Example:
1
n = 10, I pick 6.
2
3
Return 6.
Copied!

2. Implementation

(1) Binary Search
1
/* The guess API is defined in the parent class GuessGame.
2
@param num, your guess
3
@return -1 if my number is lower, 1 if my number is higher, otherwise return 0
4
int guess(int num); */
5
6
public class Solution extends GuessGame {
7
public int guessNumber(int n) {
8
int start = 1, end = n, mid = 0;
9
10
while (start <= end) {
11
mid = start + (end - start) / 2;
12
if (guess(mid) == -1) {
13
end = mid - 1;
14
}
15
else if (guess(mid) == 1) {
16
start = mid + 1;
17
}
18
else {
19
return mid;
20
}
21
}
22
return -1;
23
}
24
}
Copied!

3. Time & Space Complexity

时间复杂度O(logn), 空间复杂度O(1)