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.

题意：已有一个API，可以用来判断一个数与目标数字的大小关系，使用这个API求出目标数字

解法：使用二分法，注意数据类型溢出

1. int guess(int num);
3. class Solution {
4. public:
5. int guessNumber(int n) {
6. int low = 1;
7. int high = n;
8. int res;
9. int mid;
10. while(high > low){
11. mid = low + (high - low) / 2;
12. res = guess(mid);
13. if(res == 1) low = mid + 1;
14. else if(res == -1) high = mid - 1;
15. else return mid;
16. }
17. return low;
18. }
19. };