Go: package main

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

// Define disjoint-set forests

type vertexNo int

type vertex struct {
	number vertexNo
	rank int
	parent *vertex
}

func makeSet(number vertexNo) vertex {
	return vertex {
		number: number,
		rank: 0,
		parent: nil,
	}
}

func union(x, y *vertex) *vertex {
	return link(findSet(x), findSet(y))
}

func link(x, y *vertex) *vertex {
	if x.rank > y.rank {
		y.parent = x
		return x
	} else {
		x.parent = y
		if x.rank == y.rank {
			y.rank += 1
		}
		return y
	}
}

func findSet(v *vertex) *vertex {
	if v.parent == nil {
		return v
	}
	v.parent = findSet(v.parent)
	return v.parent
}

// Function to calculate minimum spanning tree

type matrix [][]int

type edge struct {
	one vertexNo
	two vertexNo
	weight int
}

const(
	MAX_VERTICES = 100
)

func newEdgesFromMatrix(matrix matrix) []edge {
	edges := make([]edge, 0, MAX_VERTICES * MAX_VERTICES)

	for i := 0; i < len(matrix); i++ {
		row := matrix[i]
		for j := i; j < len(row); j++ {
			if row[j] >= 0 {
				edge := edge{one: vertexNo(i+1), two: vertexNo(j+1), weight: row[j]}
				edges = append(edges, edge)
			}
		}
	}

	return edges
}

func solve(matrix matrix) int {
	nVertices := len(matrix)

	// initialize vertices
	vertices := make([]vertex, nVertices)
	for i := 0; i < nVertices; i++ {
		vertices[i] = makeSet(vertexNo(i+1))
	}

	// prepare edges sorted in the ascending order of weight
	edges := newEdgesFromMatrix(matrix)
	sort.Slice(edges, func(i, j int) bool {
		return edges[i].weight < edges[j].weight
	})

	unionCount := 0
	weightSum := 0
	for unionCount < nVertices-1 {
		for i := 0; i < len(edges); i++ {
			edge := edges[i]
			v1 := &vertices[edge.one-1]
			v2 := &vertices[edge.two-1]
			rep1 := findSet(v1)
			rep2 := findSet(v2)
			if rep1.number == rep2.number {
				continue
			}
			union(rep1, rep2)
			edges = append(edges[0:i], edges[i+1:]...)
			weightSum += edge.weight
			unionCount++
			break
		}
	}

	return weightSum
}

// main

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

	n, err := nextInt(sc)
	if err != nil {
		panic(err)
	}

	matrix := parseMatrix(n, sc)

	edges := newEdgesFromMatrix(matrix)
	sort.Slice(edges, func(i, j int) bool {
		return edges[i].weight < edges[j].weight
	})

	fmt.Println(solve(matrix))
}

func parseMatrix(n int, sc *bufio.Scanner) [][]int {
	matrix := make([][]int, n)
	for i := 0; i < n; i++ {
		matrix[i] = make([]int, n)
		for j := 0; j < n; j++ {
			x, err := nextInt(sc)
			if err != nil {
				panic(err)
			}
			matrix[i][j] = x
		}
	}
	return matrix
}

func nextText(sc *bufio.Scanner) string {
	sc.Scan()
	return sc.Text()
}

func nextInt(sc *bufio.Scanner) (int, error) {
	return strconv.Atoi(nextText(sc))
}

C++: #include <iostream>
#include <utility>
#include <vector>
#include <algorithm>


using namespace std;

int P[110];

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

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

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

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

int main(){
    int n; 
    cin >> n; 
    int w;
    init(n);
    vector< pair<int, pair<int, int> > > edges;
    for (int i = 0; i < n; i++){
        for (int j = i; j < n; j++){
            cin >> w;
            if (w != -1) edges.push_back(make_pair(w, make_pair(i, j)));
        }
    }
    sort(edges.begin(),edges.end());
    int sum = 0;
    while(!edges.empty()){
        pair<int, pair<int, int> > edge = edges[0];
        edges.erase(edges.begin());
        int cost = edge.first;
        int src = edge.second.first;
        int dst = edge.second.second;
        cout << cost << " " << src << " " << dst << endl;
        if (!is_same_set(src, dst)){
            cout << "unite " << src << " " <<  dst << " cost = " << cost << endl;
            unite(src, dst);
            sum += cost;
        }
    }
    cout << sum << endl;
}

C++: #include <iostream>
#include <climits>
#include <queue>

using namespace std;
static const int MAX=100;
static const int WHITE=1;
static const int GRAY=2;
static const int BLACK=3;

int n,M[MAX+10][MAX+10];

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

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

    d[0]=0;

    while(1)
    {
        minv=INT_MAX;
        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]!=INT_MAX)
        {
            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)? INT_MAX:e;
    }

    cout<<prim()<<endl;
    return 0;
}

C++: #include<bits/stdc++.h>
#define ll long long int
#define FOR(i,a,b) for(ll i=(ll)(a);i<(ll)(b);i++)
#define REP(i,n) FOR(i,0,n)
#define REP1(i,n) FOR(i,1,n)
 
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];

  REP(i, n) {
    d[i] = INFTY;
    p[i] = -1;
    color[i] = white;
  }

  d[0] = 0;

  while( 1 ) {
    minv = INFTY;
    u = -1;
    REP(i, n) {
      if (minv > d[i] && color[i] != black ) {
        u = i;
        minv = d[i];
      }
    }
    if (u == -1) break;
    color[u] = black;
    REP(v, n) {
      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;
  REP(i, n) {
    if (p[i] != -1) sum += M[i][p[i]];
  }
  return sum;
}

int main() {
  cin >> n;
  REP(i, n) {
    REP(j, n) {
      int e;
      cin >> e;
      M[i][j] = (e == -1) ? INFTY : e;
    }
  }
  cout << prim() << endl;
  return 0;
}

C++: #include <cstdio>
#include <iostream>
#include <cmath>
#include <ctype.h>
#include <string>
#include <sstream>
#include <iostream>
#include <algorithm>
#include <cstdlib>
#include <map>
#include <queue>
#include <utility>
#include <vector>
#include <set>
#include <list>
#include <cstring>
#include <stack>

using namespace std;

#define mod 1000000007

class UF
{
private:
	vector<int> root;

public:
	UF(int n)
	{
		root = vector<int>(n + 1, -1);
	}

	int getroot(int n)
	{
		if(root[n] == -1) return n;
		else return root[n] = getroot(root[n]);
	}

	void add(int x, int y)
	{
		x = getroot(x);
		y = getroot(y);
		if(x == y) return;
		if(x > y) swap(x, y);
		root[y] = x;
		return;
	}
};

int main()
{
	int n;
	cin >> n;
	priority_queue<pair<int, pair<int, int> > > qu;
	for(int i = 0; i < n; i++){
		for(int j = 0; j < n; j++){
			int w;
			cin >> w;
			if(w >= 0){
				qu.push(make_pair(-w, make_pair(i, j)));
			}
		}
	}
	UF uf(n);
	int ans = 0;
	while(!qu.empty()){
		int cost = -(qu.top()).first;
		int x = ((qu.top()).second).first;
		int y = ((qu.top()).second).second;
		qu.pop();
		if(uf.getroot(x) == uf.getroot(y)) continue;
		ans += cost;
		uf.add(x, y);
	}
	cout << ans << endl;
	return 0;
}