找到，返回下標；

int binary_search(const int* a, int low_position, int high_position, const int key)
{
    high_position--;
    while(low_position < high_position) {
        int mid_position = (low_position+high_position) >> 1;
        if(a[mid_position] == key) return mid_position;
        else if(a[mid_position] < key) low_position = mid_position + 1;
        else high_position = mid_position - 1;
    }
    return -1;
}

char* binary_search(const int* a, int low_position, int high_position, const int key)
{
    high_position--;
    char **result = {"success", "fail"};
    while(low_position < high_position) {
        int mid_position = (low_position+high_position) >> 1;
        if(a[mid_position] == key) return *result;
        else if(a[mid_position] < key) low_position = mid_position + 1;
        else high_position = mid_position - 1;
    }
    return *(result+1);
}

二分查找函數