This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
// 70 pts
#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>
using namespace std;
int nums;
int arr[1005];
const int mod = 1'000'007;
int memo[1005][1005][2];
int dp(int x, int mval, bool attop) {
if (x == nums) return 1;
if (memo[x][mval][attop] != -1) return memo[x][mval][attop];
int ans = 0;
if (attop) {
ans += dp(x + 1, max(arr[x], mval), 1);
ans %= mod;
ans += (arr[x] - 1) * dp(x + 1, mval, 0);
ans %= mod;
}
else {
ans += dp(x + 1, mval + 1, 0);
ans %= mod;
ans += mval * dp(x + 1, mval, 0);
ans %= mod;
}
return memo[x][mval][attop] = ans;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
cin >> nums;
for (int i = 0; i < nums; i++) cin >> arr[i];
memset(memo, -1, sizeof(memo));
cout << dp(1, 1, 1);
return 0;
}
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