[leetcode] 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.

解法一：

折半猜数字。使用一个shift去标识区间。

// 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;
while(left<n){
int mid = (n-left)/2 + left;
int g = guess(mid);
if (g == 0) return mid;
else if (g == -1) return n = mid;
else left = mid;
}
return left;
}
};