Language: C++

#include <iostream>
#include <string>
#include <vector>
#include <set>
#include <stack>
#include <queue>
#include <map>
#include <algorithm>
#include <iomanip>
#include <math.h>
using namespace std;
using ll = long long; using pll = pair<ll, ll>;
using vl = vector<ll>; using vll = vector<vl>;
//**関数リスト**//
int ctoi(char c) {
	switch (c) {
	case '0': return 0;
	case '1': return 1;
	case '2': return 2;
	case '3': return 3;
	case '4': return 4;
	case '5': return 5;
	case '6': return 6;
	case '7': return 7;
	case '8': return 8;
	case '9': return 9;
	default: return 0;
	}
}
bool pairCompare(const pll firstElof, pll secondElof)
{
	return firstElof.second < secondElof.second;
}
ll nod(ll F) {
	ll keta = 1;
	while (F / 10 > 0) {
		keta++; F /= 10;
	}
	return keta;
}
ll gcd(ll x, ll y) {
	ll r;
	if (x < y) {
		swap(x, y);
	}
	while (y > 0) {
		r = x % y;
		x = y;
		y = r;
	}
	return x;
}
ll lcm(ll x, ll y) {
	return x * y / gcd(x, y);
}
ll isPrime(ll x) {
	ll i;
	if (x < 2) {
		return 0;
	}
	else if (x == 2) {
		return 1;
	}
	else if (x % 2 == 0) {
		return 0;
	}
	else {
		for (i = 3; i * i <= x; i += 2) {
			if (x % 1 == 0) {
				return 0;
			}
		}
		return 1;
	}
}
void eratos(vl isPrime) {
	//(注)isPrimeのサイズはN+1にする！実際にはmain内に配置して使用
	ll i, j;
	for (i = 0; i < isPrime.size(); i++) {
		isPrime[i] = 1;
	}
	isPrime[0] = 0; isPrime[1] = 0;
	for (i = 2; i * i <= isPrime.size() - 1; i++) {
		if (isPrime[i] == 1) {
			j = i * 2;
			while (j <= isPrime.size() - 1) {
				isPrime[j] = 0;
				j = j + i;
			}
		}
	}
}
ll modinv(ll a, ll m) {
	ll b = m, u = 1, v = 0;
	while (b) {
		ll t = a / b;
		a -= t * b; swap(a, b);
		u -= t * v; swap(u, v);
	}
	u %= m;
	if (u < 0) u += m;
	return u;
}
ll alphabet(char C) {
	ll b = 0; ll key = 0;
	if (C == 'a') { return b; }b++;
	if (C == 'b') { return b; }b++;
	if (C == 'c') { return b; }b++;
	if (C == 'd') { return b; }b++;
	if (C == 'e') { return b; }b++;
	if (C == 'f') { return b; }b++;
	if (C == 'g') { return b; }b++;
	if (C == 'h') { return b; }b++;
	if (C == 'i') { return b; }b++;
	if (C == 'j') { return b; }b++;
	if (C == 'k') { return b; }b++;
	if (C == 'l') { return b; }b++;
	if (C == 'm') { return b; }b++;
	if (C == 'n') { return b; }b++;
	if (C == 'o') { return b; }b++;
	if (C == 'p') { return b; }b++;
	if (C == 'q') { return b; }b++;
	if (C == 'r') { return b; }b++;
	if (C == 's') { return b; }b++;
	if (C == 't') { return b; }b++;
	if (C == 'u') { return b; }b++;
	if (C == 'v') { return b; }b++;
	if (C == 'w') { return b; }b++;
	if (C == 'x') { return b; }b++;
	if (C == 'y') { return b; }b++;
	if (C == 'z') { return b; }
	return -1;
}
void bitSearch(ll n) {
	//実際にはコピーして中身を改変して使う
	ll i;
	for (i = 0; i < pow(2, n); i++) {
		ll p = i;
		for(i = 0; i < n; i++) {
			cout << p % 2; p /= 2;
		}
		cout << endl;
	}
}
void bfs(ll now) {
	//中身は毎回書き換えて使用
	queue<ll> Q;
	Q.push(now);
	ll u;
	while (!Q.empty()) {
		u = Q.front();
		Q.pop();
		//ll v=;
		//Q.push(v);
	}
}
//**定義場所**//
ll i, j, k, l, m, n, N, M, K, H, W;
ll MOD = 1000000007; ll ans = 0;
//***********//

double dp[310][310][310];
double rec(int i, int j, int k) {
	if (dp[i][j][k] >= 0) return dp[i][j][k];
	if (i == 0 && j == 0 && k == 0) return 0.0;

	double res = 0.0;
	if (i > 0) res += rec(i - 1, j, k) * i;
	if (j > 0) res += rec(i + 1, j - 1, k) * j;
	if (k > 0) res += rec(i, j + 1, k - 1) * k;
	res += N;
	res *= 1.0 / (i + j + k);

	return dp[i][j][k] = res;
}

int main() {
	cin >> N;
	int one = 0, two = 0, three = 0;
	for (int i = 0; i < N; ++i) {
		int a; cin >> a;
		if (a == 1) ++one;
		else if (a == 2) ++two;
		else ++three;
	}

	// 初期化
	memset(dp, -1, sizeof(dp));

	// 答え
	cout << fixed << setprecision(10) << rec(one, two, three) << endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:188:9: error: 'memset' was not declared in this scope
  188 |         memset(dp, -1, sizeof(dp));
      |         ^~~~~~
a.cc:11:1: note: 'memset' is defined in header '<cstring>'; this is probably fixable by adding '#include <cstring>'
   10 | #include <math.h>
  +++ |+#include <cstring>
   11 | using namespace std;