Language: C++

#include<iostream>
#include<fstream>
#include<sstream>
#include<algorithm>
#include<vector>
#include<queue>
#include<map>
#include<set>
#include<string>
#include<cmath>
#include<cstdio>
#include<cassert>
#include<iomanip>

using namespace std;

#define REP(i,m,n) for(int i=(m); i<(int)(n); i++)
#define RREP(i,m,n) for(int i=(int)(n-1); i>=m; i--)
#define rep(i,n) REP(i,0,n)
#define rrep(i,n) RREP(i,0,n)
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
#define aut(r,v) __typeof(v) r = (v)
#define each(it,o) for(aut(it,(o).begin()); it!=(o).end(); ++it)
#define reach(it,o) for(aut(it,(o).rbegin()); it!=(o).rend(); ++it)
#define fi first
#define se second
#define debug(...) {cerr<<"[L"<<__LINE__<<"] "; _debug(__VA_ARGS__);}

template<typename T1, typename T2> ostream& operator<<(ostream& o, const pair<T1, T2>& p)
{return o<<"("<<p.first<<", "<<p.second<<")";}
template<typename T>string join(const vector<T>&v, string del=", ")
{stringstream s;rep(i,v.size())s<<del<<v[i];return s.str().substr(del.size());}
template<typename T>ostream& operator<<(ostream& o, const vector<T>&v)
{if(v.size())o<<"["<<join(v)<<"]";return o;}
template<typename T>ostream& operator<<(ostream& o, const vector<vector<T> >&vv)
{int l=vv.size();if(l){o<<endl;rep(i,l){o<<(i==0?"[ ":",\n  ")<<vv[i]<<(i==l-1?" ]":"");}}return o;}
template<typename T>ostream& operator<<(ostream& o, const set<T>& st)
{vector<T> v(st.begin(),st.end());o<<"{ "<<join(v)<<" }";return o;}
template<typename T1, typename T2>ostream& operator<<(ostream& o, const map<T1, T2>& m)
{each(p,m){o<<(p==m.begin()?"{ ":",\n  ")<<*p<<(p==--m.end()?" }":"");}return o;}
inline void _debug(){cerr<<endl;}
template<class First, class... Rest>
void _debug(const First& first, const Rest&... rest){cerr<<first<<" ";_debug(rest...);}

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<vl> vvl;

const double PI = (1*acos(0.0));
const double INF = 0x3f3f3f3f;
const double INFL = 0x3f3f3f3f3f3f3f3fLL;
const double EPS = 1e-9;
const double mod = 1e9 + 7;

inline void finput(string filename) {
    freopen(filename.c_str(), "r", stdin);
}

int N;
double dp[310][310][310];

double rec(int c1, int c2, int c3){
    if(dp[c1][c2][c3] >= 0) return dp[c1][c2][c3];
    if(c1==0 && c2==0 && c3==0) return 0.0;

    double res = 0.0;
    if(c1) res += rec(c1 - 1, c2, c3) * c1;
    if(c2) res += rec(c1 + 1, c2 - 1, c3) * c2;
    if(c3) res += rec(c1, c2 + 1, c3 - 1) * c3;
    res += N;
    res *= 1.0 / (c1+c2+c3);
    return dp[c1][c2][c3] = res;
}

int main(){
    ios_base::sync_with_stdio(0);
    // finput("./input");

    cin >> N;
    int C[4];
    rep(i,N){
        int a; cin >> a;
        C[a]++;
    }
    memset(dp, -1, sizeof(dp));
    cout << fixed << setprecision(10) << rec(C[1],C[2],C[3]) << endl;
    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:89:5: error: 'memset' was not declared in this scope
   89 |     memset(dp, -1, sizeof(dp));
      |     ^~~~~~
a.cc:14:1: note: 'memset' is defined in header '<cstring>'; this is probably fixable by adding '#include <cstring>'
   13 | #include<iomanip>
  +++ |+#include <cstring>
   14 |