D-Query SQRT-Decomposition

#include <bits/stdc++.h>
#define all(x) x.begin(), x.end()
#define f(i,a,b) for(int i = (a); i <= (b); i++)
#define pb push_back
#define pii pair<int,int>

struct Query
{
    int l,r,id;
    Query(int p1, int p2, int p3) : l(p1), r(p2), id(p3) {};
    friend bool operator < (Query q1, Query q2)
    {
        if(q1.l/200 != q2.l/200) return q1.l/200 < q2.l/200;
        return q1.r < q2.r;
    }
};

int A[30005], Ans[200005], C[1000005], N, Q;
vector<Query> Queries;

int main()
{
    cin >> N;
    f(i,1,N) scanf("%d",&A[i]);
    cin >> Q;
    f(i,1,Q)
    {
        int l,r;
        scanf("%d%d",&l,&r);
        Queries.pb(Query(l,r,i));
    }
    sort(all(Queries));
    int ans = 0, a = 0, b = 0;
    for(Query q : Queries)
    {
        int l = q.l, r = q.r, id = q.id;
        while(b < r)
        {
            C[A[++b]]++;
            if(C[A[b]] == 1) ans++;
        }
        while(b > r)
        {
            if(C[A[b]] == 1) ans--;
            C[A[b--]]--;
        }
        while(a > l)
        {
            C[A[--a]]++;
            if(C[A[a]] == 1) ans++;
        }
        while(a < l)
        {
            if(a>0 && C[A[a]] == 1) ans--;
            C[A[a++]]--;
        }

        Ans[id] = ans;
    }

    f(i,1,Q) printf("%d\n", Ans[i]);
}