【模板】二分

int binary_search(int arr[],int n,int key)
{
    int low=0,high=n-1;
    while(low<=high)
    {
        int mid=low+(high-low)>>1;
        if(arr[mid]>key)
            high=mid-1;
        else if(arr[mid]<key)
            low=mid+1;
        else return mid;
    }
    return -1;
}