P1972 HH的项链（在线做法）

#include <bits/stdc++.h>
using namespace std;
using LL = long long;
const int N = 1e6 + 10;

struct tree {
    int L;
    int R;
    int cnt;
}t[N << 5];

int a[N], up[N], root[N], cnt;

int change(int pre, int l, int r, int x)
{
    int rt = ++cnt;
    t[rt] = t[pre];
    t[rt].cnt++;
    if(l >= r) return rt;

    int mid = l + r >> 1;
    if(x <= mid)
        t[rt].L = change(t[pre].L, l, mid, x);
    else    
        t[rt].R = change(t[pre].R, mid + 1, r, x);
    return rt;
}

int ask(int askl, int askr, int l, int r, int r1, int r2)
{
    if(l >= askl && r <= askr) return t[r2].cnt - t[r1].cnt;

    int ans = 0;
    int mid = l + r >> 1;
    if(mid >= askl) ans += ask(askl, askr, l, mid, t[r1].L, t[r2].L);
    if(mid < askr)  ans += ask(askl, askr, mid + 1, r, t[r1].R, t[r2].R); 
    return ans;
}

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int n;
    cin >> n;
    for(int i = 1; i <= n; i++)
    cin >> a[i];
    
    for(int i = 1; i <= n; i++) {
        root[i] = change(root[i - 1], 0, N, up[a[i]]);
        up[a[i]] = i;
    }

    int m;
    cin >> m;
    int l, r;
    for(int i = 1; i <= m; i++)
    {
        cin >> l >> r;
        cout << ask(0, l - 1, 0, N, root[l - 1], root[r]) << '\n';
    }   
}