[1.27] T3-序列问题

题目大意

题目解析

代码

#include<bits/stdc++.h>
#define M 1000000007
#define N 500005
#define L long long
using namespace std;
L mx[N],mn[N],sx[N],s[N],sm[N],sn[N];
int n;
ifstream fin("seq.in");
ofstream fout("seq.out");
L dfs(int l,int r)
{
	if(l==r) 
	 return s[l]*s[r]%M;
	int m=l+r>>1,pi=m,pj=m;
	L ans=(dfs(l,m)+dfs(m+1,r))%M;
	mx[m+1]=-M; mn[m+1]=M; 
	sm[m+1]=sn[m+1]=sx[m+1]=0;
	for(int i=m;i>=l;i--)
	{
	  mx[i]=max(mx[i+1],s[i]);
	  mn[i]=min(mn[i+1],s[i]);
	  sm[i]=(sm[i+1]+mx[i])%M;
	  sn[i]=(sn[i+1]+mn[i])%M;
	  sx[i]=(sx[i+1]+mx[i]*mn[i]%M)%M;
	}
	L rm=-M,rn=M;
	for(int j=m+1;j<=r;j++)
	{
	  rm=max(s[j],rm);
	  rn=min(s[j],rn);
	  while(pi>=l&&mx[pi]<rm) pi--;
	  while(pj>=l&&mn[pj]>rn) pj--;
	  if(pi>pj)
	   ans=(ans+(m-pi)*rm%M*rn%M+rn%M*(sm[pj+1]-sm[pi+1])%M+(sx[l]-sx[pj+1])%M)%M;
	  else 
	   ans=(ans+(m-pj)*rm%M*rn%M+rm*(sn[pi+1]-sn[pj+1])%M+(sx[l]-sx[pi+1])%M)%M;
	}
	return ans;
}
int main()
{
	fin>>n;
	for(int i=1;i<=n;i++)
	 fin>>s[i];
	L ans=dfs(1,n);
	fout<<(M+ans)%M;
}