BZOJ-2308 小z的袜子(莫队)

题目链接

题意

$n$点$m$次询问区间内随机取两个数是相同数的概率

思路

莫队入门题，对询问按块排序后更新答案，复杂度$O(n\sqrt{n})$

代码

//#pragma comment(linker, "/STACK:1024000000,1024000000")
#include <map>
#include <set>
#include <cmath>
#include <queue>
#include <stack>
#include <cstdio>
#include <vector>
#include <cassert>
#include <cstring>
#include <iostream>
#include <algorithm>

#define IOS    ios::sync_with_stdio(0),cin.tie(0);
#define DBG(x) cerr << #x << " = " << x << endl;

using namespace std;

typedef long long LL;
typedef long double LD;
typedef unsigned long long ULL;

const int inf = 0x3f3f3f3f;
const int mod = 1000000007;
const double eps = 1e-8;
const double pi  = acos(-1.0);

void file(){
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
}

const int maxn = 5e4+5;

int n,m,sz;
LL Ans,a[maxn],cnt[maxn];
struct node{
    LL l,r,id;
}Q[maxn],ans[maxn];

bool cmp(node a,node b){
    if(a.l/sz != b.l/sz)return a.l < b.l;
    return a.r < b.r;
}

void add(int x){
    Ans+=cnt[a[x]];
    cnt[a[x]]++;
}

void del(int x){
    cnt[a[x]]--;
    Ans-=cnt[a[x]];
}

void Mo(){
    sort(Q+1,Q+1+m,cmp);
    int L=1,R=1;
    cnt[a[1]]=1;//视题目而定
    for(int i=1;i<=m;i++){
        while(L < Q[i].l)del(L),L++;
        while(L > Q[i].l)L--,add(L);
        while(R < Q[i].r)R++,add(R);
        while(R > Q[i].r)del(R),R--;
        LL tmp=Q[i].r-Q[i].l+1;
        ans[Q[i].id].l=(tmp*(tmp-1))/2;
        ans[Q[i].id].r=Ans;
    }
}

namespace BakuretsuMahou{
    void Explosion(){
        scanf("%d%d",&n,&m);
        sz=sqrt(n);
        for(int i=1;i<=n;i++)scanf("%lld",&a[i]);
        for(int i=1;i<=m;i++)scanf("%lld%lld",&Q[i].l,&Q[i].r),Q[i].id=i;
        Mo();
        for(int i=1;i<=m;i++){
            if(ans[i].r == 0)puts("0/1");
            else{
                LL G=__gcd(ans[i].r,ans[i].l);
                printf("%lld/%lld\n",ans[i].r/G,ans[i].l/G);
            }
        }
    }
}

int main(){
    //IOS
    //file();
    BakuretsuMahou::Explosion();
    return 0;
}

考试周了，我为什么还是这么摸鱼，可恶。