374. Guess Number Higher or Lower

Posted 2020-10-20 __Meng

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.

猜数字，返回-1，往更小的猜，返回1往更大的猜

 int mid = (left+right)>>1    会溢出

C++（3ms）：

 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 left = 1 ;
10         int right = n ;
11         while(left <= right){
12             int mid = (right - left)/2 + left ; 
13             if (guess(mid) == 0){
14                 return mid ;
15             }else if (guess(mid) == 1){
16                 left = mid+1 ;
17             }else if (guess(mid) == -1){
18                 right = mid-1 ;
19             }
20         }
21         return 0 ;
22     }
23 };