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 <stdio.h>
#include <stdint.h>
#include <iostream>
#include <fstream>
#include <cassert>
#include <climits>

#include <vector>
#include <string>
using namespace std;

#define DEBUG (0)

#if DEBUG
#define DPRINTF(...) do { \
    printf("D(L%d) %s: ", __LINE__, __func__); \
    printf(__VA_ARGS__); \
  } while (false)
#else
#define DPRINTF(...)
#endif // DEBUG

/*
   recursive + dfs
*/

const uint8_t kMaxN = 100;
int16_t gWeightOfVij[kMaxN][kMaxN];
int gNumOfVertices;


void printAllWeights()
{
  DPRINTF("Number of vertices %d\n", gNumOfVertices);

  for (int i = 0; i < gNumOfVertices; i++)
  {
    for (int j = 0; j < gNumOfVertices; j++)
    {
      if (j == 0)  DPRINTF("");
      printf(" %d", gWeightOfVij[i][j]);
    }
    cout << endl;
  }
}


vector<int16_t> getConnectedVs(int16_t v, const bool visited[])
{
  vector<int16_t> vertices;

  for (int i = 0; i < gNumOfVertices; i++)
  {
    if (gWeightOfVij[v][i] > 0 && !visited[i])
      vertices.push_back(i);
  }

#if 0
  if (vertices.size() > 0)
  {
    for (int i = 0; i < vertices.size(); i++)
    {
      if (i == 0)
        DPRINTF("v from %d to :", v);

      printf(" %d", vertices[i]);
    }

    cout << endl;
  }
#endif

  return vertices;
}


int minSpanningTreeRecursive(int16_t currentV, const bool visited[], string path)
{
  assert(0 <= currentV && currentV < gNumOfVertices);

  bool updatedVisited[gNumOfVertices];

  for (int i = 0; i < gNumOfVertices; i++)
    updatedVisited[i] = visited[i];

  updatedVisited[currentV] = true;

  // This function ensures all of elements are unvisited
  vector<int16_t> nextVs = getConnectedVs(currentV, updatedVisited);

  if (nextVs.size() == 0)
    return 0;

  //string minPath;
  int minV = -1;
  int minScore = INT_MAX;
  string currentPath = to_string(currentV);

  if (path.size())
    currentPath = path + " > " + currentPath;

  for (int i = 0; i < nextVs.size(); i++)
  {
    const int16_t nextV = nextVs[i];
    const int score = gWeightOfVij[currentV][nextV] + minSpanningTreeRecursive(nextV, updatedVisited, currentPath);

    if (minScore > score)
    {
      minScore = score;
      minV = nextV;
    }

  }

  {
    string minPath = "[" + to_string(currentV) + "] > " + to_string(minV);

    if (path.size())
      minPath = path + " > " + minPath;

    DPRINTF("score %d (currentV %d)\n", minScore, currentV);
    DPRINTF("  %s\n", minPath.c_str());
  }

  return minScore;
}


int minSpanningTree()
{
  int16_t startV = 1;
  bool visited[gNumOfVertices];

  for (int i = 0; i < gNumOfVertices; i++)
    visited[i] = false;

  return minSpanningTreeRecursive(startV, visited, "");
}


int main()
{
  cin >> gNumOfVertices; cin.ignore();

  DPRINTF("Number of vertices %d\n", gNumOfVertices);

  for (int i = 0; i < gNumOfVertices; i++)
  {
    for (int j = 0; j < gNumOfVertices; j++)
      cin >> gWeightOfVij[i][j];

    cin.ignore();
  }

#if DEBUG
  DPRINTF("Number of vertices %d\n", gNumOfVertices);
  printAllWeights();
#endif // DEBUG

  cout << minSpanningTree() << endl;

  return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <cstring>
#define MAX 105
#define INF 1000000
using namespace std;

int n;
int M[MAX][MAX];
int d[MAX];
int p[MAX];
int color[MAX];//0 表示未进入 -1 表示在之中 1 表示已出去

int prim() {
	int u,min,i,j;
	d[0]=0;
	p[0]=-1;
	while(1) {
		min=INF;
		u=-1;
		for(i=0; i<n; i++) {
			if(min>d[i] && color[i]!=1) {
				u=i;
				min=d[i];
			}
		}
		if(u==-1) break;
		color[u]=1;
		for(j=0; j<n; j++) {
			if(color[j]!=1 && M[u][j]!=INF) {
				if(d[j]>M[u][j]) {
					d[j]=M[u][j];
					color[j]=-1;
					p[j]=u;
				}
			}
		}
	}
	int sum=0;
	for(i=0; i<n; i++) {
		if(p[i] != -1) sum+=M[i][p[i]];
	}
	return sum;
}

int main() {
	int i,j,a;
	memset(color,0,sizeof(color));
	memset(d,INF,sizeof(d));
	memset(p,-1,sizeof(p));
	scanf("%d",&n);
	for(i=0; i<n; i++) {
		for(j=0; j<n; j++) {
			scanf("%d",&a);
			if(a==-1) a=INF;
			M[i][j]=a;
		}
	}
	cout<<prim()<<endl;
	return 0;
}

C++: #include <cstdio>
#include <iostream>
#include <queue>
#include <utility>

using namespace std;

const int NIL = 2001;
const int MAX = 100;
int G[MAX][MAX];
int color[MAX];
int W[MAX];
int n;

const int WHITE = 0;
const int GRAY = 1;
const int BLACK = 2;

void MST() {
	priority_queue<pair<int, int>> PQ;
	PQ.push(make_pair(0, 0));
	W[0] = 0;
	color[0] = BLACK;
	while (!PQ.empty()) {
		int u = PQ.top().second;
		PQ.pop();
		color[u] = BLACK;
		for (int i = 0; i < n; ++i) {
			if (G[u][i] != -1 && G[u][i] < W[i] && color[i] != BLACK) {
				PQ.push(make_pair(G[u][i] * -1, i));
				W[i] = G[u][i];
//				cout << u << ":" << i << ":" << G[u][i] << endl;
			}
		}
	}
}

int main() {

	scanf("%d", &n);
	for (int i = 0; i < n; ++i) {
		W[i] = NIL;
		color[i] = WHITE;
	}

	for (int i = 0; i < n; ++i) {
		for (int j = 0; j < n; ++j) {
			int a;
			scanf("%d", &a);
			G[i][j] = a;
		}
	}

	MST();

	cout << endl;

	int sum = 0;
	for (int i = 0; i < n; ++i) {
//		cout << i << ":" << W[i] << endl;
		sum += W[i];
	}
	cout << sum << endl;

	return 0;
}