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 <algorithm>
#include <vector>

using namespace std;

int G[110][110];
int cost[1000],src[1000],dst[1000];

int P[10010];
int root(int a);
int is_same_set(int a,int b);
void unite(int a,int b);

int main() {
	int N,a;
	cin >> N;
	for(int i=0; i<N;i++) {
		for(int j=0;j<N;j++) {
			cin >> a;
			G[i][j] = a;
		}
	}
	int hcon = 1;
	int M = 0;
	pair<int,int> edges[10000];

	for(int r=0;r<N;r++) {
		for (int c=0;c<hcon;c++) {
			if(G[r][c] > -1) {
				cost[M] = G[r][c];
				src[M] = r;
				dst[M] = c;
				edges[M] = make_pair(cost[M],M);
				M+=1;
			}
		}
		hcon++;
	}
	sort(edges,edges+M);



	for(int i=0;i<N;i++) P[i] = i;

	int answer = 0;
	for(int i=0;i<M;i++) {
		if(!is_same_set(src[edges[i].second],dst[edges[i].second])) {
			unite(src[edges[i].second],dst[edges[i].second]);
			answer += edges[i].first;
			//cout << answer << endl;
		}
	}
	cout << answer << endl;;
}

int root(int a) { 
	if (P[a] == a) return a; 
	return (P[a] = root(P[a])); 
}
int is_same_set(int a, int b) { 
	if(root(a) == root(b)) {
		return 1;
	} else {
		return 0;
	}
}
void unite(int a, int b) { 
	P[root(a)] = root(b);
}

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]=WHITE;
    }

    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 <stdio.h>
#include <iostream>
#include <utility>
#include <algorithm>
#include <math.h>
#include <vector>
#define N 10000
using namespace std;

class edge
{
  public:
    int i, j, weigt;
    bool operator()(edge e1, edge e2)
    {
        return e1.weigt < e2.weigt;
    }
};
edge e[N];
vector<int> adj[N];
int n;
int c = 0;
int f[N] = {};
int g[N] = {};

void setGraphNum(int v, int i)
{
    g[v] = i;
    f[v]++;
    vector<int>::iterator itr = adj[v].begin();
    while (itr != adj[v].end())
    {
        if (f[*itr] != c)
            setGraphNum(*itr, i);
        itr++;
    }
}
void CalcGraphNum()
{
    int k = 1;
    for (int i = 0; i < n; i++)
    {
        if (f[i] != c)
            setGraphNum(i, k++);
    }
}
int main()
{
    cin >> n;
    int k = 0, tmp = 0, sum = 0;
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            cin >> tmp;
            if (j < i + 1 || tmp < 0)
                continue;
            e[k].i = i;
            e[k].j = j;
            e[k].weigt = tmp;
            k++;
        }
    }
    sort(e, e + k, edge());
    for (int i = 0; i < n; i++)
        g[i] = i;
    for (int i = 0; i < k; i++)
    {
        if (g[e[i].i] == g[e[i].j])
            continue;
        adj[e[i].i].push_back(e[i].j);
        adj[e[i].j].push_back(e[i].i);
        sum += e[i].weigt;
        //cout << e[i].i << " " << e[i].j << " " << e[i].weigt << "\n";
        if (++c == n - 1)
            break;
        CalcGraphNum();
    }

    cout << sum << "\n";
    return 0;
}

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;
}