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 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）：

// Forward declaration of guess API.
// @param num, your guess
// @return -1 if my number is lower, 1 if my number is higher, otherwise return 0
int guess(int num);

class Solution {
public:
    int guessNumber(int n) {
        int left = 1 ;
        int right = n ;
        while(left <= right){
            int mid = left + (right - left)/2 ;
            int t = guess(mid) ;
            if (t == 0)
                return mid ;
            else if (t == -1)
                right = mid - 1;
            else if (t == 1)
                left = mid + 1 ;
        }
    }
};