compress v2

//int a[N];

void compress()
{
    vector<int> b(a+1,a+1+n);
    sort(b.begin(),b.end());
    for(int i=1; i<=n; i++)
    {
        int x = a[i];
        int k = lower_bound(b.begin(),b.end(),x)-b.begin()+1;
        a[i] = k;
    }
}