This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <bits/stdc++.h>
using namespace std;
#define ll long long
const int nx=1e4+5, mod=1e6+7;
ll n, a[nx], h[nx], res, dp[2][nx];
int main()
{
cin.tie(NULL)->sync_with_stdio(false);
cin>>n;
for (int i=1; i<=n; i++) cin>>a[i], h[i]=max(h[i-1], a[i]);
for (int i=1; i<=n; i++) dp[0][i]=1;
a[0]=1;
for (int i=1; i<=n; i++)
{
int c=i%2, p=1-c;
res=(res+(dp[p][h[n-i]])*(a[n-i+1]-1))%mod;
for (int j=1; j<=n; j++) dp[c][j]=(dp[p][j]*j+dp[p][j+1])%mod; //cout<<"debug "<<i<<' '<<j<<' '<<dp[c][j]<<'\n';
}
cout<<(res+1)%mod;
}
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