Submission #486129

#	Time	Username	Problem	Language	Result	Execution time	Memory
486129		MilosMilutinovic	Kangaroo (CEOI16_kangaroo)	C++14	100 / 100	25 ms	19040 KiB

kangaroo

#include <bits/stdc++.h>

using namespace std;

const int mod = 1e9 + 7;

int add(int x, int y) {
  x += y;
  return x >= mod ? x - mod : x;
}

int mul(int x, int y) {
  return (long long) (x * y) % mod;
}

int sub(int x, int y) {
  x -= y;
  return x < 0 ? x + mod : x;
}

const int N = 2005;

int a, b, dif, dp[N][N], ft[N][N];
bool was[N][N];

long long Solve(int n, int id) {
  if (was[n][id]) {
    return dp[n][id];
  }
  was[n][id] = true;
  if (id == 1) {
    return dp[n][id] = ft[n][n - dif];
  }
  else if (id == 2) {
    return dp[n][id] = add(ft[n][n - dif], ft[n - 1][n - 1 - dif]);
  } else {
    return dp[n][id] = sub(sub(mul(2, Solve(n, id - 1)), Solve(n, id - 2)), Solve(n - 2, id - 2));
  }
}

void Precalc(int n) {
  ft[2][2] = 1; // pocetni slucaj
  for (int i = 3; i <= n; i++) {
    if (i % 2 == 1) {
      int sum = 0;
      for (int j = 2; j < n; j++) {
        sum = add(sum, ft[i - 1][j]);
      }
      ft[i][2] = add(ft[i][2], sum);
    }
    for (int j = 3; j <= i; j++) {
      if (i % 2 == 1) {
        ft[i][j] = sub(ft[i][j - 1], ft[i - 1][j - 1]);
      } else {
        ft[i][j] = add(ft[i][j - 1], ft[i - 1][j - 1]);
      }
    }
  }
}

long long resi(int n) {
  Precalc(n);
  return Solve(n, a);
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n >> a >> b;
  if (a > b) {
    swap(a, b);
  }
  dif = n - b;
  cout << resi(n) << '\n';
  return 0;
}

• Subtask 1: 6 / 6
• Subtask 2: 30 / 30
• Subtask 3: 15 / 15
• Subtask 4: 49 / 49