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;
using ll = long long;
const ll MOD = 1e6 + 7;
int n;
vector<int> a;
const int N = 1e4 + 100;
ll dp[N][N];
int main ()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n;
a.resize( n );
for ( auto& el : a )
cin >> el;
for ( int i=0; i<N; i++ )
for ( int j=0; j<N; j++ )
dp[i][j] = 0;
for ( int i=1; i<=n; i++ )
dp[0][i] = 1;
for ( int i=1; i<=n; i++ )
for ( int j=1; j<=n; j++ )
dp[i][j] = ( j*dp[i-1][j] + ( j<n ? dp[i-1][j+1] : 0 ) ) % MOD;
ll ans = 1;
int maxi = 0;
for ( int i=0; i<n; i++ )
{
for ( int j=1; j < a[i]; j++ )
ans = ( ans + dp[n-1-i][ max( maxi, j ) ] ) % MOD;
maxi = max( maxi, a[i] );
}
cout << ans;
return 0;
}
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