Submission #259300

#	Time	Username	Problem	Language	Result	Execution time	Memory
259300		Bruteforceman	Ruins 3 (JOI20_ruins3)	C++11	100 / 100	2609 ms	16132 KiB

ruins3

#include <bits/stdc++.h>
using namespace std;
const int maxn = 605;
const long long mod = 1e9 + 7;
long long ways[maxn];
int a[maxn];
int var;
long long mem[maxn][maxn];
long long C[maxn * 2][maxn * 2];
long long fac[] = {1, 1, (mod + 1) / 2};
long long calc(int cur, int take) {
  long long ans = 0;
  if(cur == 1) {
    for(int i = 0; i <= 2; i++) {
      if(take + i == var + 1) {
        return (C[take + i][i] * (fac[2 - i])) % mod;
      }
    }
    return 0;
  }
  if(mem[cur][take] != -1) return mem[cur][take];
  for(int i = 0; i <= 2; i++) {
    if(take + i <= var - cur + 1) {
      long long add = (calc(cur - 1, take + i) * C[take + i][i]) % mod;
      ans += (add * (fac[2 - i])) % mod;
      ans %= mod;
    }
  }
  return mem[cur][take] = ans;
}
long long dp[maxn][maxn];

int main() {
  int n;
  cin >> n;
  for(int i = 1; i <= n; i++) cin >> a[i];
  for(int i = 0; i <= n + n; i++) {
    C[i][0] = 1;
    for(int j = 1; j <= i; j++) {
      C[i][j] = (C[i - 1][j] + C[i - 1][j - 1]) % mod;
    }
  }
  for(int i = 1; i <= n; i++) {
    memset(mem, -1, sizeof mem);
    var = i;
    ways[i] = calc(var, 0);
  }
  memset(mem, -1, sizeof mem);
  dp[n][0] = 1;
  for(int cur = n - 1; cur >= 0; cur--) {
    for(int bound = 0; bound <= n; bound++) {
      dp[cur][bound] = (1LL * dp[cur + 1][bound] * (n - bound - cur)) % mod;
      for(int i = 0; i < bound; i++) {
        int free = a[cur + 1] - (n - bound + (cur + 1));
        if(free >= bound - i) {
          long long add = ((C[free][bound - i] * ways[bound - i] % mod) * dp[cur + 1][i]) % mod;
          dp[cur][bound] += add;
          dp[cur][bound] %= mod;
        }
      }
    }
  } 
  cout << dp[0][n] << endl;
  return 0;
}

• Subtask 1: 6 / 6
• Subtask 2: 52 / 52
• Subtask 3: 42 / 42