#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
const int inf_int = 1e9 + 100;
const ll inf_ll = 1e15;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef long double dbl;
#define pb push_back
const double pi = 3.1415926535898;
#define dout if(debug) cout
#define fi first
#define se second
#define sp setprecision
#define sz(a) (int(a.size()))
#define all(a) a.begin(),a.end()
bool debug = 0;
const int MAXN = 1e5 + 100;
const int LOG = 20;
const int mod = 1e9 + 7;
const int MX = 1e6 + 100;
typedef long long li;
const li MOD = 1000000000949747713ll;
int N, S, E;
int dp[2010][1020][2][2];
ll get(int i, int comp, int le, int ri) {
if (comp < 0)
return 0;
if (2 * comp + 1 > N)
return 0;
if (dp[i][comp][le][ri] != -1)
return dp[i][comp][le][ri];
ll &res = dp[i][comp][le][ri];
res = 0;
if (i == N - 1) {
if (comp == 0) {
res = 1;
//check
}
return res;
}
if (i == S) {
//just
res += get(i + 1, comp, 1, ri);
//conn
res += get(i + 1, comp - 1, 1, ri) * comp;
} else if (i == E) {
res += get(i + 1, comp, le, 1);
res += get(i + 1, comp - 1, le, 1) * comp;
} else {
//just create
res += get(i + 1, comp + 1, le, ri);
//connect
res += get(i + 1, comp - 1, le, ri) * (1ll * comp * (comp - 1)) % mod;
if (le > 0) {
//connect
res += get(i + 1, comp - 1, le, ri) * comp;
}
if (ri > 0) {
//connect
res += get(i + 1, comp - 1, le, ri) * comp;
}
}
res%=mod;
return res;
}
void solve() {
memset(dp,-1,sizeof dp);
cin >> N >> S >> E;
S--; E--;
cout << get(0,0,0,0)<<"\n";
}
signed main() {
#ifdef zxc
debug = 1;
freopen("../input.txt", "r", stdin);
#else
#endif //zxc
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cout.setf(ios::fixed);
cout.precision(20);
int t = 1;
while (t--)
solve();
dout << (1.0 * clock() / CLOCKS_PER_SEC) << endl;
}
Compilation message
kangaroo.cpp: In function 'll get(int, int, int, int)':
kangaroo.cpp:38:33: error: cannot bind non-const lvalue reference of type 'll& {aka long long int&}' to an rvalue of type 'll {aka long long int}'
ll &res = dp[i][comp][le][ri];
~~~~~~~~~~~~~~~~~~^
