Submission #127451

#	Time	Username	Problem	Language	Result	Execution time	Memory
127451		RockyB	Tents (JOI18_tents)	C++17	0 / 100	33 ms	35704 KiB

tents

#include <bits/stdc++.h>

#define rep(a, b, c) for (int a = (b); (a) <= (c); a++)
#define per(a, b, c) for (int a = (b); (a) >= (c); a--)

#define nl '\n'
#define ioi exit(0);

const int mod = (int)1e9 + 7;
const int N = (int)1e4;

using namespace std;

void add(int &x, int y) {
  x += y;
  if (x >= mod) x -= mod;
  if (x < 0) x += mod;
}
int sum(int x, int y) {
  add(x, y);
  return x;
}
int bp(int x, int y) {
  int res = 1;
  while (y) {
    if (y & 1) res = res * 1ll * x % mod;
    x = x * 1ll * x % mod, y >>= 1;
  }
  return res;
}

int n, m;
int f[N], rv[N];

int dp[3001][3001];
int cnk(int x, int y) {
  return (f[x] * 1ll * rv[x - y] % mod) * 1ll * rv[y] % mod;
}
int ank(int x, int y) {
  return f[x] * 1ll * rv[x - y] % mod;
}
int calc(int v = n, int free = m) {
  if (!v) return 1;
  if (~dp[v][free]) return dp[v][free];
  int res = calc(v - 1, free);
  // put < or > or ^ or v
  if (free > 0) add(res, calc(v - 1, free - 1) * 1ll * free * 4 % mod);
  // connect v and ^
  // if (must > 0) add(res, calc(v + 1, free, must - 1) * 1ll * must % mod);
  // open v
  // if (free > 0) add(res, calc(v + 1, free - 1, must + 1) * 1ll * free % mod);
  // connect > and <
  if (free > 1) add(res, calc(v - 1, free - 2) * 1ll * cnk(free, 2) % mod);
  return dp[v][free] = res;
}

int place(int put) {
  int res = 1;
  rep(i, 0, put - 1) {
    res = res * 1ll * cnk(n - 2 * i, 2);
  }
  return res;
}
int main() {
  #ifdef IOI
    freopen ("in.txt", "r", stdin);
  #endif
  f[0] = rv[0] = 1;
  rep(i, 1, N - 1) {
    f[i] = f[i - 1] * 1ll * i % mod;
    rv[i] = bp(f[i], mod - 2);
  }
  cin >> n >> m;
  //cout << calc()
  memset(dp, -1, sizeof(dp));
  int ans = sum(-1, calc());
  rep(i, 1, n / 2) {
    int take = cnk(m, i) * 1ll * place(i) % mod; 
    // ans = 0;
    if (i <= m) add(ans, take * 1ll * calc(n - 2 * i, m - i) % mod);
    //cerr << ans << ' ' << take << ' ' << calc(n - 2 * i, m - i) << nl;
  }
  cout << ans;
  ioi
}

/*

5116 0
3720 1
450 2

*/

• Subtask 1: 0 / 48
• Subtask 2: 0 / 52