| # | Time | Username | Problem | Language | Result | Execution time | Memory |
|---|---|---|---|---|---|---|---|
| 1123884 | ljtunas | Kangaroo (CEOI16_kangaroo) | C++17 | 104 ms | 98628 KiB |
// author : anhtun_hi , nqg
#include <bits/stdc++.h>
#define ll long long
#define ii pair<ll, ll>
#define fi first
#define se second
#define all(x) x.begin(), x.end()
#define reset(h, v) memset(h, v, sizeof h)
#define Forv(i, a) for(auto& i : a)
#define For(i, a, b) for(int i = a; i <= b; ++i)
#define Ford(i, a, b) for(int i = a; i >= b; --i)
#define c_bit(i) __builtin_popcountll(i)
#define Bit(mask, i) ((mask >> i) & 1LL)
#define onbit(mask, i) ((mask) bitor (1LL << i))
#define offbit(mask, i) ((mask) &~ (1LL << i))
#define Log2(x) (63 - __builtin_clzll(x))
#define TIME (1.0 * clock() / CLOCKS_PER_SEC)
using namespace std;
namespace std {
template <typename T, int D>
struct _vector : public vector <_vector <T, D - 1>> {
static_assert(D >= 1, "Dimension must be positive!");
template <typename... Args>
_vector(int n = 0, Args... args) : vector <_vector <T, D - 1>> (n, _vector <T, D - 1> (args...)) {}
};
template <typename T> struct _vector <T, 1> : public vector <T> {
_vector(int n = 0, const T& val = T()) : vector <T> (n, val) {}
};
template <class A, class B> bool minimize(A &a, B b){return a > b ? a = b, true : false;}
template <class A, class B> bool maximize(A &a, B b){return a < b ? a = b, true : false;}
}
const int dx[] = {0, 0, +1, -1}, dy[] = {-1, +1, 0, 0}, LOG = 20, base = 311, inf = 1e9 + 5;
const int MAXN = 5e3 + 5;
const ll mod = 1e9 + 7;
const ll oo = 1e18;
//#define int long long
namespace Mod{
inline void add(int &x, const int &y) { if((x += y) >= mod) x -= mod; }
inline void sub(int &x, const int &y) { if((x -= y) < 0) x += mod; }
inline void Mul(int &x, const int &y) { x = 1LL * x * y % mod; }
inline int mul(const int &x, const int &y) { return 1LL * x * y % mod; }
inline int sum(const int &x, const int &y) { int res = x + y; if(res >= mod) res -= mod; return res; }
inline int diff(const int &x, const int &y) { int res = x - y; if(res < 0) res += mod; return res; }
} using namespace Mod;
int n, s, t, dp[MAXN][MAXN];
int solve(int i, int j)
{
if (j < 0) return 0;
if (i > n) return j == 1;
if (~dp[i][j]) return dp[i][j];
int res = 0, cost = (i > s) + (i > t);
if (i == s || i == t) {
add(res, solve(i + 1, j + 1));
add(res, solve(i + 1, j));
} else {
add(res, mul(solve(i + 1, j + 1), (j + 1 - cost)));
add(res, mul(solve(i + 1, j - 1), (j - 1)));
}
return dp[i][j] = res;
}
void Solve() {
cin >> n >> s >> t;
memset(dp, -1, sizeof dp);
if (n == 2) cout << 1 << '\n';
else cout << solve(1, 0) << '\n';
}
signed main() {
cin.tie(0) -> sync_with_stdio(0);
if(fopen("kangaroo.inp", "r")) {
freopen("kangaroo.inp", "r", stdin);
freopen("kangaroo.out", "w", stdout);
}
int t = 1;
// cin >> t;
while(t --) Solve();
return 0;
}