莫队

查询区间每个数出现的个数，离线算法，O(nsqrt(n))

一、普通莫队

题目链接

https://www.luogu.com.cn/problem/P2709

题目大意

题目代码

#include<iostream>
#include<algorithm>
#include<cmath>
#define ll long long
const int N = 5e4 + 5;
using namespace std;
int n,m,k,block;
struct node{
    int l,r,id;
}q[N];
ll a[N],cnt[N],ans[N],sum;
bool cmp(const node& a,const node& b){
    if(a.l / block != b.l / block) return a.l < b.l;
    return a.r < b.r;
}
void add(int x){
    sum -= cnt[x] * cnt[x];
    ++cnt[x];
    sum += cnt[x] * cnt[x];
}
void del(int x){
    sum -= cnt[x] * cnt[x];
    --cnt[x];
    sum += cnt[x] * cnt[x];
}
int main()
{
    scanf("%d%d%d",&n,&m,&k);
    block = sqrt(n);
    for(int i = 1;i <= n;++i) scanf("%d",&a[i]);
    for(int i = 1;i <= m;++i) scanf("%d%d",&q[i].l,&q[i].r),q[i].id = i;
    sort(q + 1,q + m + 1,cmp);
    for(int i = 1,l = 1,r = 0;i <= m;++i){
        while(l > q[i].l) add(a[--l]);
        while(r < q[i].r) add(a[++r]);
        while(l < q[i].l) del(a[l++]);
        while(r > q[i].r) del(a[r--]);
        ans[q[i].id] = sum;
    }
    for(int i = 1;i <= m;++i){
        printf("%lld\n",ans[i]);
    }
    return 0;
}