#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef pair<int, int> pi;
#define debug(x) cerr << #x << ": " << x << endl;
#define debug2(x, y) debug(x) debug(y);
#define repn(i, a, b) for(int i = (int)(a); i < (int)(b); i++)
#define rep(i, a) for(int i = 0; i < (int)(a); i++)
#define all(v) v.begin(), v.end()
#define mp make_pair
#define pb push_back
#define lb lower_bound
#define ub upper_bound
#define fi first
#define se second
#define sq(x) ((x) * (x))
const int MOD = 1e6 + 7;
const int mxN = 1e4 + 5;
template<class T> T gcd(T a, T b){ return ((b == 0) ? a : gcd(b, a % b)); }
int dp[2][mxN][2];
int main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
//freopen("input.in", "r", stdin);
//freopen("output.out", "w", stdout);
int n;
cin >> n;
vi a(n);
rep(i, n) cin >> a[i];
memset(dp, 0, sizeof(dp));
dp[0][1][1] = 1;
dp[0][1][0] = 0;
repn(i, 1, n){
int cur = i & 1, prev = 1 - cur;
repn(j, 1, i + 1){
//turn a 1 into a 0
(dp[cur][j][0] += (dp[prev][j][1] * min(j, a[i] - 1)) % MOD) %= MOD;
if((j + 1) < a[i]) (dp[cur][j + 1][0] += dp[prev][j][1] % MOD) %= MOD;
//update the 0 guy
(dp[cur][j][0] += (dp[prev][j][0] * j) % MOD) %= MOD;
(dp[cur][j + 1][0] += dp[prev][j][0]) %= MOD;
//update the 1 guy
(dp[cur][max(j, a[i])][1] += dp[prev][j][1]) %= MOD;
}
memset(dp[prev], 0, sizeof(dp[prev]));
}
int ans = 0;
repn(i, 1, n + 1) ans = (ans + dp[(n - 1) & 1][i][0]) % MOD;
cout << ans + 1 << endl;
return 0;
}
/*
Things to look out for:
- Integer overflows
- Array bounds
- Special cases
Be careful!
*/
# |
Verdict |
Execution time |
Memory |
Grader output |
1 |
Correct |
0 ms |
512 KB |
Output is correct |
2 |
Correct |
0 ms |
512 KB |
Output is correct |
3 |
Correct |
0 ms |
512 KB |
Output is correct |
4 |
Correct |
1 ms |
512 KB |
Output is correct |
5 |
Correct |
0 ms |
512 KB |
Output is correct |
6 |
Correct |
1 ms |
512 KB |
Output is correct |
# |
Verdict |
Execution time |
Memory |
Grader output |
1 |
Correct |
1 ms |
512 KB |
Output is correct |
2 |
Correct |
0 ms |
512 KB |
Output is correct |
3 |
Correct |
0 ms |
512 KB |
Output is correct |
# |
Verdict |
Execution time |
Memory |
Grader output |
1 |
Correct |
1 ms |
512 KB |
Output is correct |
2 |
Correct |
1 ms |
544 KB |
Output is correct |
3 |
Correct |
1 ms |
640 KB |
Output is correct |
# |
Verdict |
Execution time |
Memory |
Grader output |
1 |
Correct |
1 ms |
512 KB |
Output is correct |
# |
Verdict |
Execution time |
Memory |
Grader output |
1 |
Correct |
1 ms |
512 KB |
Output is correct |
2 |
Correct |
1 ms |
512 KB |
Output is correct |
3 |
Correct |
1 ms |
512 KB |
Output is correct |
# |
Verdict |
Execution time |
Memory |
Grader output |
1 |
Correct |
3 ms |
512 KB |
Output is correct |
2 |
Correct |
3 ms |
512 KB |
Output is correct |
3 |
Correct |
3 ms |
512 KB |
Output is correct |
# |
Verdict |
Execution time |
Memory |
Grader output |
1 |
Correct |
8 ms |
512 KB |
Output is correct |
2 |
Correct |
8 ms |
512 KB |
Output is correct |
3 |
Correct |
10 ms |
512 KB |
Output is correct |
# |
Verdict |
Execution time |
Memory |
Grader output |
1 |
Incorrect |
694 ms |
580 KB |
Output isn't correct |
# |
Verdict |
Execution time |
Memory |
Grader output |
1 |
Incorrect |
145 ms |
560 KB |
Output isn't correct |
2 |
Halted |
0 ms |
0 KB |
- |
# |
Verdict |
Execution time |
Memory |
Grader output |
1 |
Incorrect |
544 ms |
708 KB |
Output isn't correct |
2 |
Halted |
0 ms |
0 KB |
- |