标题B3

简单！

Code

#include "guess.h"
#include <iostream>
using namespace std;

int guess(int n) {
    int low = 1;
    int high = n;
    while (true) {
        if (low == high) return low;
        int mid = (low + high)/2;
        int result = ask(mid);
        if (result == 0) return mid;
        else if (result < 0) high = mid-1;
        else low = mid+1;
    }
}