莫队乱搞--BZOJ2038: [2009国家集训队]小Z的袜子(hose)

$n \leq 50000$的$\leq 50000$的数字序列，$m \leq 50000$个询问，每次问一个区间中随机拿两次（不放回）拿到相同数字的概率，以既约分数形式输出。

莫队入门。把询问按“同块排$r$、不同块排$l$”的顺序，依靠左右端点两个指针跑来跑去依次回答。

 //#include<iostream>
 #include<cstring>
 #include<cstdlib>
 #include<cstdio>
 //#include<queue>
 //#include<time.h>
 //#include<complex>
 #include<algorithm>
 #include<stdlib.h>
 using namespace std;

 int n,m,lq,tot;
 #define maxn 50011
 #define maxm 255
 int bel[maxn],a[maxn],cnt[maxn];
 struct Ques{int l,r,id;}q[maxn];
 bool cmp(const Ques a,const Ques b) {return bel[a.l]==bel[b.l]?a.r<b.r:a.l<b.l;}

 #define LL long long
 LL ss;
 struct Ans{LL a,b;}ans[maxn];
 LL gcd(LL a,LL b) {return b?gcd(b,a%b):a;}
 void modify(int p,int type) {ss-=(LL)cnt[a[p]]*cnt[a[p]]; cnt[a[p]]+=type; ss+=(LL)cnt[a[p]]*cnt[a[p]];}

 int main()
 {
     scanf("%d%d",&n,&lq);
     m=; for (int i=;i<=n;i++) bel[i]=(i-)/m+; tot=bel[n];
     for (int i=;i<=n;i++) scanf("%d",&a[i]);
     for (int i=;i<=lq;i++) scanf("%d%d",&q[i].l,&q[q[i].id=i].r);
     sort(q+,q++lq,cmp);

     int L=,R=; ss=;
     for (int i=;i<=lq;i++)
     {
         while (L<q[i].l) modify(L,-),L++;
         while (L>q[i].l) modify(L-,),L--;
         while (R<q[i].r) modify(R+,),R++;
         while (R>q[i].r) modify(R,-),R--;
         Ans &now=ans[q[i].id];
         now.a=ss-(q[i].r-q[i].l+);
         now.b=(q[i].r-q[i].l)*1ll*(q[i].r-q[i].l+);
         LL g=gcd(now.a,now.b); now.a/=g; now.b/=g;
     }

     for (int i=;i<=lq;i++) printf("%lld/%lld\n",ans[i].a,ans[i].b);
     return ;
 }