二分查找

template<typename comparable>
int binarySearch(const vector<comparable> &a, const comparable &x){
    int low = 0, high = a.size() - 1;

    while(low <= high)
    {
        int center = (high + low)/2;
        if(x < a[center])
            high = center - 1;
        else if(x > a[center])
            low = center + 1;
        else
            return center;
    }

    return -1;
}