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

int n, M[MAX][MAX];

int prim() {
    int d[n], p[n];
    int color[n];
    
    for(int i=0; i<n; i++) {
        color[i] = WHITE;
        d[i] = INFTY;
        p[i] = -1;
    }
    d[0] = 0;
    
    while(1) {
        int u=-1;
        int mincost = INFTY;
        for (int i=0; i<n; i++) {
            if (color[i] != BLACK && d[i] < mincost) {
                mincost = d[i];
                u = i;
            }
        }
        
        if (mincost == INFTY) break;
        
        color[u] = BLACK;
        
        for (int v=0; v<=n; v++) {
            if (color[v] != BLACK && M[u][v] != INFTY) {
                if(M[u][v] < d[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() {
    scanf("%d", &n);
    for (int i=0; i<n; i++) {
        for (int j=0; j<n; j++) {
            int tmp;
            scanf("%d", &tmp);
            M[i][j] = (tmp == -1) ? INFTY : tmp;
        }
    }
    
    int sum = prim();
    printf("%d\n", sum);
    
    return 0;
}

C++: #include <bits/stdc++.h>
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, 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;
      M[i][j] = (e == -1) ? INFTY : e;
    }
  }

  cout << prim() << endl;

  return 0;

}

C++: // Ref : https://book.mynavi.jp/ec/products/detail/id=35408

#include <iostream>
using namespace std;
const static int NMAX = 100;
const static int VMAX = 2000;
const static int INF = VMAX + 1;
int n, G[NMAX][NMAX];

// prim's algorithm for Minimum Spanning Tree (MST)
int prim() {
	bool state[NMAX];
	int distance[NMAX];
	for (int i = 0; i < n; i++) {
		state[i] = false;
		distance[i] = INF;
	}
	distance[0] = 0;

	while (true) {
		int u, mincost = INF;
		for (int i = 0; i < n; i++) {
			if (!state[i] && (distance[i] < mincost)) {
				u = i;
				mincost = distance[i];
			}
		}
		if (mincost == INF)	break;
		state[u] = true;

		for (int i = 0; i < n; i++) {
			if (!state[i] && (G[u][i] < distance[i])) {
				distance[i] = G[u][i];
			}
		}
	}

	int sum = 0;
	for (int i = 0; i < n; i++) {
		if (distance[i] != INF) sum += distance[i];
	}

	return sum;
}

int main(void) {
	cin >> n;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			cin >> G[i][j];
			if (G[i][j] == -1) G[i][j] = INF;
		}
	}

	cout << prim() << endl;

	return 0;
}

C++: //By Vlgd
#include<bits/stdc++.h>
#define INF 20000000
using namespace std;
const int N=100;
int G[N][N],n;
int vis[N]={0},dis[N]={0};
void solve(){
	int ans=0;
	for(int i=0;i<n;i++) dis[i]=INF;
	dis[0]=0;
	while(1){
		int mv=INF,u=-1;
		for(int i=0;i<n;i++){
			if(mv>dis[i]&&!vis[i]){
				mv=dis[i];
				u=i;
			}
		}
		if(u==-1) break;
		vis[u]=1;
		ans+=dis[u];

		for(int v=0;v<n;v++){
			if(!vis[v]&&G[u][v]!=-1){
				if (dis[v]>G[u][v])	dis[v]=G[u][v];
			}
		}
	}
	printf("%d\n",ans);	
}
main(){
	scanf("%d",&n);
	for (int i=0;i<n;i++) for (int j=0;j<n;j++) scanf("%d",&G[i][j]);
	solve();
}

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, 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;
      M[i][j] = (e == -1) ? INFTY : e;
    }
  }

  cout << prim() << endl;

  return 0;
}

C++: #include<iostream>
#include<algorithm>
using namespace std;
#define MAX_E 100005
#define MAX_V 10005

struct edge{int u, v, cost;};

bool comp(const edge& e1, const edge& e2){
	return e1.cost < e2.cost;
}
int V, E;
edge es[MAX_E];
int par[MAX_V];
int _rank[MAX_V];

void init(int x){
	for(int i = 0; i < x; i++){
		par[i] = i;
		_rank[i] = 0;
	}
}

int find(int x){
	return par[x] == x ? x : par[x] = find(par[x]);
}

void unite(int x, int y){
	x = find(x);
	y = find(y);
	
	if(_rank[x] < _rank[y]){
		par[x] = y;
	} else{
		par[y] = x;
		if(_rank[x] == _rank[y]) _rank[x]++;
	}
}

bool same(int x, int y){
	return find(x) == find(y);
}

int kruskal(){
	sort(es, es + E, comp);
	init(V);
	int res = 0, count = 0;
	for(int i = 0; i < E; i++){
		edge e = es[i];
		if(!same(e.u, e.v)){
			unite(e.u,e.v);
			res += e.cost;
			count++;
		}
	}
	return res;
}

int main(){
	int s, t, w, x, y = 0;
	
	cin>>V;
	
	for(int i = 1; i <= V; i++){
		for(int j = 1; j <= V; j++){
			cin>>x;
			if(x != -1 && i <= j){
				es[y].u = i;
				es[y].v = j;
				es[y++].cost = x;
			}
		}
	}
	
	E = y;
	
	cout<<kruskal()<<endl;
	
	return 0;
	
}