Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

//util
var sc = bufio.NewScanner(os.Stdin)

func nextStr() string {
	sc.Scan()
	return sc.Text()
}

func nextInt() int {
	sc.Scan()
	i, e := strconv.Atoi(sc.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func printList(list []int) {
	n := len(list)
	buf := bufio.NewWriter(os.Stdout)
	for _, v := range list[0 : n-1] {
		buf.Write([]byte(fmt.Sprintf("%d ", v)))
	}
	buf.Write([]byte(fmt.Sprintf("%d\n", list[n-1])))
	buf.Flush()
}

func max(a, b int) int {
	if a > b {
		return a
	} else {
		return b
	}
}

func min(a, b int) int {
	if a > b {
		return b
	} else {
		return a
	}
}

//solve
const (
	MAX   = 100
	INFTY = (1 << 21)
	WHITE = 0
	GRAY  = 1
	BLACK = 2
)

//V: グラフの頂点全体の集合
//T: MSTに属する頂点の集合
var (
	n     int
	M     [MAX][MAX]int //重みを記録する隣接行列
	color [MAX]int
	p     [MAX]int //MSTにおける頂点の親を記録
	d     [MAX]int //Tに属する頂点と、V−Tに属する頂点を繋ぐ辺の中で、重みが最小のものを記録
)

func prim() int {
	var u, minv int
	//初期化
	for i := 0; i < n; i++ {
		d[i] = INFTY
		p[i] = -1
		color[i] = WHITE
	}

	d[0] = 0

	for {
		//頂点uを決めていく
		//つまり、Tに属する頂点と、V−Tに属する頂点を繋ぐ辺の中で、重みが最小のものを決める
		minv = INFTY
		u = -1
		for i := 0; i < n; i++ {
			//未訪問かつ、辺の重みが最小となるuを選ぶ
			if minv > d[i] && color[i] != BLACK {
				u = i
				minv = d[i]
			}
		}

		if u == -1 {
			break
		}

		color[u] = BLACK
		for v := 0; v < n; v++ {
			//未訪問かつ、辺が存在するとき
			if color[v] != BLACK && M[u][v] != INFTY {
				if d[v] > M[u][v] {
					d[v] = M[u][v]
					p[v] = u
					color[v] = GRAY
				}
			}
		}
	}

	sum := 0
	for i := 0; i < n; i++ {
		if p[i] != -1 {
			sum += M[i][p[i]]
		}
	}
	return sum
}

func main() {
	sc.Split(bufio.ScanWords)
	n = nextInt()

	for i := 0; i < n; i++ {

		for j := 0; j < n; j++ {
			tmp := nextInt()
			if tmp == -1 {
				tmp = INFTY
			}
			M[i][j] = tmp
		}

	}

	fmt.Println(prim())

}

C++: #include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using vi = vector<int>;    using vvi = vector<vi>;
using vb = vector<bool>;   using vvb = vector<vb>;
using vl = vector<ll>;     using vvl = vector<vl>;
using vd = vector<double>; using vvd = vector<vd>;

#define REP(i,n) for(auto i = 0 * n; i < (n); ++i)
#define ALL(c) (c).begin(), (c).end()
#define FOR(i,s,e) for (ll i = s; i < (ll)e; i++)
#define TEN(x) ((ll)1e##x)


struct Edge{ uint32_t to, id; };
typedef vector<vector<Edge>> Graph;

template<class T> struct Prim {
	Prim(const Graph & es, const vector<T> & ws) : g(es.size()) {
		typedef uint32_t U;
		typedef tuple<T, U, U, U> P;
		U n = es.size();
		vector<U> unexp(n, 1);
		priority_queue<P, vector<P>, greater<P>> q; q.emplace(0, n, 0, 0);
		while (!q.empty()) {
			T w; U u, v, i; tie(w, u, v, i) = q.top(); q.pop();
			if (unexp[v]) {
				unexp[v] = 0;
				if (u != n) { g[u].push_back({ v, i }); g[v].push_back({ u, i }); cost += w; }
				for (auto e : es[v]) if (unexp[e.to]) q.emplace(ws[e.id], v, e.to, e.id);
			}
		}
	}
	Graph g; T cost = 0;
};

int main() {
#ifdef INPUT_FROM_FILE
	ifstream cin("sample.in");
	ofstream cout("sample.out");
#endif
	cin.tie(0);
	ios_base::sync_with_stdio(false);
	cout << fixed << setprecision(50);

	uint32_t n; cin >> n;
	Graph g(n);
	vl w;
	REP(i, n) REP(j, n) {
		ll x; cin >> x;
		if (x != -1) {
			g[i].push_back({j, w.size()});
			g[j].push_back({i, w.size()});
			w.push_back(x);
		}
	}
	Prim<ll> mst(g, w);
	cout << mst.cost << endl;
}

C++: #include <iostream>
#include <cstdio>
#include <algorithm>
#include <utility>
#include <vector>
using namespace std;
#define reps(i,s,n) for(int i = s; i < n; i++)
#define rep(i,n) reps(i,0,n)
#define fi first
#define se second
typedef long long ll;
typedef vector<ll> vec;
typedef vector<vec> mat;
typedef pair<ll,ll> pa; 
#define mp make_pair;


int N,M,H,W,K,A,B;
string S;
string alpha = "abcdefghijklmnopqrstuvwxyz";
string ALPHA = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

int main() {
  cin>>N;
  mat a(N,vec(N));
  rep(i,N) rep(j,N) cin>>a.at(i).at(j);
  class union_find{
        vector<long> parent, tree_size;
    public:
        union_find(unsigned long _n) : parent(_n), tree_size(_n, 1){
            for(unsigned long i = 0; i < _n; ++i)parent[i] = i;
        }
        ll find(ll x){
            while(parent[x] != x)x = parent[x] = parent[parent[x]];
            return x;
        }
        void merge(ll a, ll b){
            a = find(a);
            b = find(b);
            if(tree_size[a] < tree_size[b])swap(a, b);
            tree_size[a] += tree_size[b];
            parent[b] = a;
            return;
        }
	bool same(ll a, ll b){
	    a = find(a);
	    b = find(b);
	    return a == b;
	}
  };
  union_find uf(N);
  vector<pair<ll,ll> > edge(0);
  rep(i,N){
    reps(j,i+1,N){
      if(a[i][j] != -1){
	edge.push_back(pa(a[i][j],i*N+j));
      }
    }
  }
  sort(edge.begin(),edge.end());
  ll anse=0,ans=0;
  rep(i,edge.size()){
    pa e = edge[i];
    if(uf.same(e.se/N,e.se%N)) continue;
    ans += e.fi;
    uf.merge(e.se/N,e.se%N);
    ++anse;
    if(anse == N-1) break;
  }
  if(anse<N-1) cout<<"impossible"<<endl;
  cout<<ans<<endl;
  //uf.mergeのように使う
  //ufはMAX_N+1でとるか、代入時に0-indexedにする
}

C++: #include <iostream>
using namespace std;
#define MAX 100
#define INFTY (1<<21)
#define WHITE 0
#define GRAY 1
#define BLACK 2

int n, M[MAX][MAX];

int prim() {
    int u, minv;
    int d[MAX], p[MAX], color[MAX];

    for(int i=0; i<n; i++) {
        d[i] = INFTY;
        p[i] = -1;
        color[i] = WHITE;
    }

    d[0] = 0;

    while(1) {
        minv = INFTY;
        u = -1;
        for(int i=0; i<n; i++) {
            if(minv >d[i] && color[i]!=BLACK) {
                u = i;
                minv = d[i];
            }
        }
        if(u==-1) {
            break;
        }
        color[u] = BLACK;
        for(int v=0; v<n; v++) {
            if(color[v]!=BLACK && M[u][v]!=INFTY) {
                if(d[v]>M[u][v]) {
                    d[v] = M[u][v];
                    p[v] = u;
                    color[v] = GRAY;
                }
            }
        }
    }
    int sum = 0;
    for(int i=0; i<n; i++) {
        if(p[i]!=-1) {
            sum += M[i][p[i]];
        }
    }

    return sum;
}

int main() {
    cin >> n;

    for(int i=0; i<n; i++) {
        for(int j=0; j<n; j++) {
            int e;
            cin >> e;
            if(e==-1) {
                M[i][j] = INFTY;
            }
            else {
                M[i][j] = e;
            }
        }
    }

    cout << prim() << endl;

    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
static const int INF   = (1 << 21);
static const int WHITE = 0;
static const int GRAY  = 1;
static const int BLACK = 2;
static const int NIL   = -1;

int n;

void prim(vector<vector<int>> &A) {
  vector<int> color(n, WHITE);
  vector<int> d(n, INF);
  vector<int> p(n, 0);
  d[0] = 0;
  p[0] = -1;

  while (true) {
    int mincost = INF;
    int u       = -1;

    for (int i = 0; i < n; i++) {
      if (color[i] != BLACK && d[i] < mincost) {
        mincost = d[i];
        u       = i;
      }
    }
    if (mincost == INF) { break; }
    color[u] = BLACK;

    for (int v = 0; v < n; v++) {
      if (color[v] != BLACK && A[u][v] != -1) {
        if (A[u][v] < d[v]) {
          d[v]     = A[u][v];
          p[v]     = u;
          color[v] = GRAY;
        }
      }
    }
  }
  int res = 0;
  for (int i = 0; i < n; i++) {
    res += d[i];
  }
  cout << res << endl;
}

int main() {
  cin >> n;
  vector<vector<int>> A(n, vector<int>(n));

  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      cin >> A[i][j];
    }
  }
  prim(A);
}