Go: package main

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

// I/O
type Scanner struct {
	sc *bufio.Scanner
}

func NewScanner() *Scanner {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	sc.Buffer(make([]byte, 1024), int(1e+9))
	return &Scanner{sc}
}

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

func (s *Scanner) nextInt() int {
	i, e := strconv.Atoi(s.nextStr())
	if e != nil {
		panic(e)
	}
	return i
}

func (s *Scanner) nextFloat() float64 {
	f, e := strconv.ParseFloat(s.nextStr(), 64)
	if e != nil {
		panic(e)
	}
	return f
}

func (s *Scanner) nextRuneSlice() []rune {
	return []rune(s.nextStr())
}

func (s *Scanner) nextIntSlice(n int) []int {
	res := make([]int, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextInt()
	}
	return res
}

func (s *Scanner) nextFloatSlice(n int) []float64 {
	res := make([]float64, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextFloat()
	}
	return res
}

// Arithmetic
func max(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m < i {
			m = i
		}
	}
	return m
}

func min(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m > i {
			m = i
		}
	}
	return m
}

func abs(x int) int {
	if x > 0 {
		return x
	}
	return -x
}

func pow(x, y int) int {
	res := 1
	for i := 0; i < y; i++ {
		res *= x
	}
	return res
}

func ceil(a, b int) int {
	if a%b == 0 {
		return a / b
	} else {
		return a/b + 1
	}
}

// Sort
type RuneSlice []rune

func (a RuneSlice) Len() int           { return len(a) }
func (a RuneSlice) Less(i, j int) bool { return a[i] < a[j] }
func (a RuneSlice) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }

const INF = 1 << 60

// Main
func main() {
	sc := NewScanner()
	wtr := bufio.NewWriter(os.Stdout)
	N := sc.nextInt()
	G := make([][]int, N)
	for i := 0; i < N; i++ {
		G[i] = make([]int, N)
		for j := 0; j < N; j++ {
			G[i][j] = sc.nextInt()
		}
	}
	d := make([]int, N)
	for i := 0; i < N; i++ {
		d[i] = INF
	}
	seen := make([]bool, N)
	d[0] = 0
	ans := 0
	for {
		cur := 0
		minCost := INF
		for i := 0; i < N; i++ {
			if !seen[i] && d[i] < minCost {
				minCost = d[i]
				cur = i
			}
		}
		if minCost == INF {
			break
		}
		seen[cur] = true
		ans += d[cur]
		for i := 0; i < N; i++ {
			cost := G[cur][i]
			if cost != -1 && cost < d[i] {
				d[i] = cost
			}
		}
	}

	fmt.Fprintln(wtr, ans)
	wtr.Flush()
}

C++: #include <iostream>
#include <vector>
#include <queue>
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[u][v]?????°???u??°v???????????????
int M[MAX][MAX];

struct segment{
    //??????
    int start;
    //??????
    int end;
    //??????
    int dis;

    segment(int s,int e,int d):start(s),end(e),dis(d){}

    bool operator < (const segment& p)const {
        return dis<p.dis;
    }
};

int prim(){
    //????????¶???
    int u=0;
    //??¨?????°???MST?????¶???V?????¶??????
    int p[MAX];
    //??¨?????°???v???????????¶???WHITE???GRAY???BLACK
    int color[MAX];
    //??¨?????°????????\T?????¶??????V-T?????¶?????????????????????????°????????????????
    priority_queue<segment,vector<segment>> d;

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

    while(true){
        bool ok=true;
        for(int i=0;i<n;i++)
            if(color[i]==WHITE){
                ok= false;
                break;
            }
        if(ok)
            break;

        color[u]=BLACK;
        for(int i=0;i<n;i++)
            if(i!=u&&color[i]==WHITE&&M[u][i]!=INFTY){
                color[i]=GRAY;
                d.push(segment(u,i,M[u][i]));
            }
        u=d.top().end;
        p[u]=d.top().start;
        d.pop();
    }

    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;
}

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

int n, M[MAX][MAX];

int prim(){
	int d[MAX], p[MAX], color[MAX];
	for (int i = 0; i < n; i++){
		d[i] = INF;
		p[i] = -1;
		color[i] = WHITE;
	}
	d[0] = 0;
	while (1){
		int u = -1;
		int minv = INF;
		for (int v = 0; v < n; v++){
			if (minv > d[v] && color[v] != BLACK){
				minv = d[v];
				u = v;
			}
		}
		if (u == -1)break;
		color[u] = BLACK;
		for (int v = 0; v < n; v++){
			if (color[v] != BLACK && M[u][v] != INF){
				if (d[v] > M[u][v]){
					d[v] = M[u][v];
					p[v] = u;
					color[v] = GLAY;
				}
			}
		}
	}
	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;
	int e;
	for (int i = 0; i < n; i++){
		for (int j = 0; j < n; j++){
			cin >> e;
			M[i][j] = (e == -1) ? INF : e;
		}
	}
	cout << prim() << endl;
}

C++: #include <cstdio>
#include <vector>
#include <map>
#include <algorithm>

#define rep(i, n) for(int i = 0; i < (n); ++i)

using namespace std;

int n;
int g[100][100];

vector<pair<int, int> > computeSpanningTree(std::vector<pair<int, pair<int, int> > >& edges){
  class UnionFind{
  public:
    UnionFind(int n){
      for(int i = 0; i < n; ++i){
        parent.push_back(i);
      }
      rank.resize(n, 0);
    }

    bool same(int a, int b){
      return find(a) == find(b);
    }

    void unite(int a, int b){
      int p = find(a), q = find(b);

      // assume that p != q

      if(rank[p] > rank[q]){
        parent[q] = p;
      }
      else{
        parent[p] = q;
        
        if(rank[p] == rank[q]){
          ++rank[q];
        }
      }
    }

  private:
    int find(int a){
      return a == parent[a] ? a : parent[a] = find(parent[a]);
    }

  private:
    vector<int> parent, rank;
  };

  class Indexer{
  public:
    int add(int p){
      if(indexer.find(p) == indexer.end()){
        int new_id = indexer.size();
        indexer[p] = new_id;
      }
      return indexer[p];
    }

  private:
    std::map<int, int> indexer;
  };

  sort(edges.begin(), edges.end());

  Indexer id;
  UnionFind uf(edges.size() * 2);

  vector<pair<int, int> > result;
  for(int i = 0; i < edges.size(); ++i){
    int from = edges[i].second.first;
    int to = edges[i].second.second;

    int a = id.add(from);
    int b = id.add(to);

    if(!uf.same(a, b)){
      result.push_back(make_pair(from, to));
      uf.unite(a, b);
    }
  }

  return result;
}

int main(){
	scanf("%d", &n);
	rep(i, n){
		rep(j, n){
			scanf("%d", &g[i][j]);
		}
	}

	std::vector<pair<int, pair<int, int> > > gr;
	rep(i, n){
		for(int j = i; j < n; ++j){
			if(g[i][j] != -1){
				gr.push_back(make_pair(g[i][j], make_pair(i, j)));
			}
		}
	}
	vector<pair<int, int> > edge = computeSpanningTree(gr);
	int s = 0;
	rep(i, edge.size()){
		s += g[edge[i].first][edge[i].second];
	}
	printf("%d\n", s);
	return 0;
}