CEOI 2020, Day 1, Fancy Fence

fancyfence.cpp

#include <bits/stdc++.h>
#define mod 1000000007
 
using namespace std;
 
stack <pair<int,int>> S,S2;
int n,v[100005],i,dr[100005],st[100005],x,Max,st2[100005];
long long suma[100001],total1,val1, val2,total2,nr;
pair <int,int > sor[100005];
 
 
long long bin(long long  x){
    if(x > mod){
	x %= mod;
    }
    return ((x*(x+1))/2)%mod;
}
long long rectCount(long long x, long long y){
    return (bin(x)*bin(y))%mod;
}
 
 
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin>>n;
for(i=1;i<=n;i++){
    cin>>v[i];
    sor[i].first = v[i];
    sor[i].second = i;
    dr[i] = n + 1;
}
for(i=1;i<=n;i++){
    cin>>x;
    suma[i] = suma[i-1] + x;
}
for(i=1;i<=n;i++){
    x = v[i];
    while(S.empty() == false && x<S.top().first){
        dr[S.top().second] = i;
        S.pop();
    }
    S.push({x,i});
}
while(S.empty() == false){
    S.pop();
}
for(i=n;i>=1;i--){
    x = v[i];
    while(S.empty() == false && x <S.top().first){
        st[S.top().second] = i;
        S.pop();
    }
    while(S2.empty() == false && x<=S2.top().first){
        st2[S2.top().second] = i;
        S2.pop();
    }
    S2.push({x,i});
    S.push({x,i});
}
for(i=1;i<=n;i++){
    nr = suma[dr[sor[i].second]-1] - suma[st[sor[i].second]];
    total1 = (total1 + rectCount(nr,sor[i].first));
    if(total1 > mod){
        total1 -= mod;
    }
    Max = max(v[st2[sor[i].second]],v[dr[sor[i].second]]);
    total2 = (total2 + rectCount(nr,Max)) % mod;
}
if(total1 < total2)
    total1 += mod;
cout << total1 - total2;
    return 0;
}