This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#pragma GCC target ("avx2")
#pragma GCC optimize ("O3")
#pragma GCC optimize ("unroll-loops")
#include<bits/stdc++.h>
#include<math.h>
using namespace std;
typedef long long int ll;
typedef long double ld;
typedef pair<ll, ll> pl;
#define K first
#define V second
#define G(x) ll x; cin >> x;
#define GD(x) ld x; cin >> x;
#define GS(s) string s; cin >> s;
#define EX(x) { cout << x << '\n'; exit(0); }
#define A(a) (a).begin(), (a).end()
#define F(i, l, r) for (ll i = (l); i < r; ++i)
#define NN 10010
#define M 1000007 // 998244353
ll dp[NN][NN];
int main(){
// freopen("a.in", "r", stdin);
// freopen("a.out", "w", stdout);
ios_base::sync_with_stdio(false); cin.tie(0);
cout << fixed << setprecision(20);
fill(dp[0], dp[0]+NN, 1ll);
F(i, 1, NN) F(l, 1, NN-1) {
dp[i][l] = (l * dp[i-1][l] + dp[i-1][l+1])%M;
}
ll mx = 1;
ll answer = 1;
G(n) for (ll left = n-1; left >= 0; --left) {
G(x)
if (x <= mx) {
// max didn't change, so normal
answer = (answer + (x-1) * dp[left][mx])%M;
} else {
// max changes at the last second;
assert(mx + 1 == x);
answer = (answer + mx * dp[left][mx])%M;
}
mx = max(mx, x);
}
cout << answer << '\n';
}
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