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.

我們正在玩一個猜數字遊戲。 遊戲規則如下：
我從 1 到 n 選擇一個數字。 你需要猜我選擇了哪個數字。
每次你猜錯了，我會告訴你這個數字是大了還是小了。
你調用一個預先定義好的接口 guess(int num)，它會返回 3 個可能的結果（-1，1 或 0）：

-1 : 我的數字比較小
 1 : 我的數字比較大
 0 : 恭喜！你猜對了！

示例 :

輸入: n = 10, pick = 6
輸出: 6

8ms

 1 class Solution {
 2     func guessNumber(_ n: Int, _ pick: Int) -> Int {
 3         if pick == 0 || pick == n {return n}
 4         var left:Int = 1
 5         var right:Int = n
 6         var mid:Int = 0
 7         while(left < right)
 8         {
 
 9             var mid = left + (right - left)/2
10             if pick == mid
11             {
12                 return mid
13             }
14             else if pick < mid
15             {
16                right = mid
17             }
18             else
19             {
20                 left = mid
21             }
22         }
23         return mid
24     }
25 }

C++版本：

 1 // Forward declaration of guess API.
 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 class Solution {
 7 public:
 8     int guessNumber(int n) {
 9         int low=1;
10         while(low<=n){
11             int mid=low+(n-low)/2;
12             int res=guess(mid);
13             if(res==0) return mid;
14             else if(res==-1) n=mid-1;
15             else low=mid+1;
16         }
17         return -1;
18     }
19 };

[Swift]LeetCode374. 猜數字大小 | Guess Number Higher or Lower