Go: package main

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

const BIG_COST = 3000

const (
	ST_NOT_YET = iota
	ST_JOINED
)

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)

	nNodes := scanInt(sc)
	costs := make([][]int, nNodes)

	for r := 0; r < nNodes; r++ {
		costs[r] = make([]int, nNodes)
		for c := 0; c < nNodes; c++ {
			costs[r][c] = scanInt(sc)
		}
	}

	states := make([]int, nNodes)
	minCosts := make([]int, nNodes)

	for i := 1; i < nNodes; i++ {
		minCosts[i] = BIG_COST
	}

	for {
		var u int
		minCost := BIG_COST

		for i := 0; i < nNodes; i++ {
			if states[i] != ST_JOINED && minCosts[i] < minCost {
				minCost = minCosts[i]
				u = i
			}
		}

		// 全てのノードが MST に JOIN できたので終了。
		if minCost == BIG_COST {
			break
		}

		states[u] = ST_JOINED

		for v := 0; v < nNodes; v++ {
			cost := costs[u][v]

			// MST内の u と MST外の v との辺の重みをそれぞれ minCosts[v] として
			// 保存しておく。このうち辺の重みが最小となる v が次のループで MST に JOIN する。
			if states[v] != ST_JOINED && cost != -1 && cost < minCosts[v] {
				minCosts[v] = cost
			}
		}
	}

	sum := 0
	for _, cost := range minCosts {
		sum += cost
	}
	fmt.Println(sum)
}

func scanInt(sc *bufio.Scanner) int {
	sc.Scan()
	v, _ := strconv.Atoi(sc.Text())
	return v
}

C++: #if 0
AOJ??????accepted??????????????????
???????????????60??????????????§?????????

pair???????????????????????¨
????????¨?????????pair?????????????????????????????¨?????????????????????????????????
#endif

#include <iostream>
#include <algorithm>
#include <utility>
using namespace std;

int P[10010];
void init(int N){//?????????
  for (int i=0; i<=N; ++i)
    P[i]=i;
}

int root(int a){//a???root????±???????
  if(P[a]==a)
    return a;
  return (P[a]=root(P[a]));
}

bool is_same_set(int a,int b){//??????set?????????
  return root(a)==root(b);
}

void unite(int a,int b){
  P[root(a)]=root(b);
}

int main(){
  int N;
  cin>>N;
  int A[N][N];
  
  for(int i=0; i<N;i++){//????????????
    for(int j=0;j<N;j++){
      cin>>A[j][i];
    }
  }

  int M = 0; // ????????????????????°
  int num=N*N;
  int cost[num];
  int src[num],dst[num];
  for (int r=0; r<N;r++){
    for (int c=0; c<r;c++){
      if (A[r][c] > -1) {
	cost[M] = A[r][c];
	src[M] = r;
	dst[M] = c;
	M += 1;
      }
    }
  }

  pair<int,int> edge[100000];//?§??????????????????¢????????????
  for(int i=0; i<M;i++){
    edge[i]=make_pair(cost[i],i);
  }
  sort(edge,edge+M);

  int edges[M][3];
  for(int i=0; i<M;i++){//?????????????????????
    edges[i][0]=edge[i].first;
    edges[i][1]=src[edge[i].second];
    edges[i][2]=dst[edge[i].second];
  }
  /*
  check!
  for(int i=0; i<M;i++){
    cout<<"news cost="<<edges[i][0]<<" ,"<<edges[i][1]<<"-"<<edges[i][2]<<endl;
  }
  */




  init(N);
  int sum=0;
  for(int i=0; i<M;i++){//???????????????????????????
    if(is_same_set(edges[i][1],edges[i][2])==0){
      unite(edges[i][1],edges[i][2]);
      sum=sum+edges[i][0];
    }
  }
    cout<<sum<<endl;
}

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

int n, M[MAX][MAX];

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

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

  // 頂点0から始めるため
  d[0] = 0;

  while (true)
  {
    mincost = INFTY;
    u = -1;
    // 初回は頂点0が選ばれる
    // 隣接する頂点を探す処理で、重みdが設定されていれば
    // 最小コストの頂点uが選ばれる
    for (int i = 0; i < n; i++)
    {
      if (mincost > d[i] && color[i] != BLACK)
      {
        u = i;
        mincost = d[i];
      }
    }

    if (u == -1)
    {
      break;
    }

    // 最小全域木の頂点に選ばれた
    color[u] = BLACK;

    for (int v = 0; v < n; v++)
    {
      if (color[v] != BLACK && M[u][v] != INFTY)
      {
        // 最小全域木に選ばれていないかつ、uに対して辺がある頂点v
        if (d[v] > M[u][v])
        {
          // 頂点uとvの辺の重みに更新する
          d[v] = M[u][v];
          p[v] = u;
          color[v] = GRAY;
        }
      }
    }
  }

  // 最小全域木の辺の重みの総和
  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;
      M[i][j] = (e == -1) ? INFTY : e;
    }
  }

  cout << prim() << endl;

  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

struct UnionFind {
  vector<int> data;
  UnionFind() {}
  UnionFind(int n) : data(n, -1) {}
  bool same(int x, int y) { return root(x) == root(y); }
  int root(int x) { return (data[x] < 0) ? x : data[x] = root(data[x]); }
  int rank(int x) { return -data[root(x)]; }
  bool unite(int x, int y) {
    x = root(x);
    y = root(y);
    if (x != y) {
      if (data[y] < data[x]) swap(x, y);
      data[x] += data[y];
      data[y] = x;
    }
    return x != y;
  }
};
template<typename T> struct Graph {
  struct Edge { int s, t; T cost; };
  int sz;
  vector<Edge> es;
  Graph(int n = 0) : sz(n) {}

  void addEdge(int u, int v, T w) {
    sz = max(sz, max(u, v) + 1);
    es.push_back({u, v, w});
  }
  T kruskal() {
    UnionFind uf(sz);
    sort(begin(es), end(es), [](Edge e1, Edge e2) {
      return e1.cost < e2.cost;
    });
    T res = 0;
    for (auto&& e : es) {
      if (uf.unite(e.s, e.t)) res += e.cost;
    }
    return res;
  }
};

int main() {
  int n; cin >> n;
  vector<vector<int>> table(n, vector<int>(n));
  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < n; ++j) {
      cin >> table[i][j];
    }
  }
  Graph<int> g;
  for (int i = 0; i < n; ++i) {
    for (int j = i + 1; j < n; ++j) {
      if (table[i][j] != -1) g.addEdge(i, j, table[i][j]);
    }
  }
  cout << g.kruskal() << endl;
  return 0;
}

C++: #include <iostream>
using namespace std;

int n;
int M[100][100];

int prim(){
  int d[100]; //最小コスト
  int color[100];
  int p[100];
  for(int i=0;i<n;i++){
    d[i] = (1 << 21);
    color[i] = 0;
    p[i] = -1;
  }
  d[0] = 0;
  p[0] = 0;

  int mincost;
  int u;
  int cost = 0;

  while(true){
    mincost = (1 << 21);
    for(int i=0;i<n;i++){
      if(color[i] != 2 && d[i] < mincost){
        mincost = d[i];
        u = i;
      }
    }

    if(mincost == (1 << 21)) break;

    color[u] = 2;
    cost += mincost;

    // uと隣接する頂点についてdの更新
    for(int i=0;i<n;i++){
      if(M[u][i] != -1 && color[i] != 2 && M[u][i] < d[i]){
        d[i] = M[u][i];
        p[i] = u;
        color[i] = 1;
      }
    }
  }
  return cost;
}

int main(){
  cin >> n;
  for(int i=0;i<n;i++){
    for(int j=0;j<n;j++){
      cin >> M[i][j];
    }
  }
  cout << prim() << endl;
  return 0;
}