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<algorithm>
using namespace std;
int matrix[100][100];
pair<int,int> edges[100];
int cost[100];
int src[100];
int dst[100];
int p[10010];
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,amount=0,sum=0;
    
    cin>>n;
    for(int i=0;i<n;i++)
    {
        for(int j=0;j<n;j++)
        {
            cin>>matrix[i][j];
        }
    }
    
    for(int i=0;i<n;i++)
    {
        for(int j=0;j<=i;j++)
        {
            if(matrix[i][j]!=-1)
            {
                cost[amount]=matrix[i][j];
                src[amount]=i;
                dst[amount]=j;
                amount++;
            }
        }
    }
    for(int i=0;i<amount;i++)
    {
        edges[i]=make_pair(cost[i],i);
    }

	init(n);
    
    sort(edges,edges+amount);
    
    for(int i=0;i<amount;i++)
    {
        if(is_same_set(src[edges[i].second()],dst[edges[i].second()]))
			continue;
		unite(src[edges[i].second()],dst[edges[i].second()]);
		sum+=edges[i].first();
    }
	cout<<sum<<endl;
}

C++: #include <iostream>
using namespace std;

const int INF = 1 << 20;
const int MAX_N = 110;

int main() {
  int cost[MAX_N][MAX_N];
  int mincost[MAX_N];
  bool used[MAX_N];
  int n, a, res;

  cin >> n;

  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < n; ++j) {
      cin >> a;
      cost[i][j] = a == -1 ? INF : a;
    }
  }

  for (int i = 0; i < n; ++i) {
    mincost[i] = INF;
    used[i] = false;
  }

  mincost[0] = 0;
  res = 0;

  while (true) {
    int v = -1;
    for (int u = 0; u < n; ++u) {
      if (!used[u] && (v == -1 || mincost[u] < mincost[v])) v = u;
    }

    if (v == -1) break;

    used[v] = true;
    res += mincost[v];

    for (int u = 0; u < n; ++u) {
      mincost[u] = min(mincost[u], cost[v][u]);
    }
  }

  cout << res << endl;
  
  return 0;
}

C++: #include <iostream>
#include <vector>
#include <numeric>
#include <limits>

using namespace std;

const int N = 100;

enum {
    WHITE,
    GRAY,
    BLACK,
};

int main() {
    int A[N][N];

    int n;
    cin >> n;
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            int a;
            cin >> a;
            A[i][j] = a == -1 ? numeric_limits<int>::max() : a;
        }
    }

    // ????????????
    vector<int> color(n, WHITE); // ??????????¨??????¶???
    vector<int> d(n, numeric_limits<int>::max());
    vector<int> p(n, -1);

    d[0] = 0;
    while (1) {
        int minv = numeric_limits<int>::max();
        int 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 && A[u][v] != numeric_limits<int>::max()) {
                if (d[v] > A[u][v]) {
                    d[v] = A[u][v];
                    p[v] = u;
                    color[v] = GRAY;
                }
            }
        }
    }
    int sum;
    for (int i = 0; i < n; ++i) {
        if (p[i] != -1)
            sum += A[i][p[i]];
    }
    cout << sum << endl;

    return 0;
}

C++: #include <iostream>
using namespace std;

const long MAX = 100;
const long INFTY = (1<<21);
const long WHITE = 0;
const long GRAY = 1;
const long BLACK = 2;

long n, M[MAX][MAX];

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

	// d: minimum dist. p: parent

	for (long i = 0; i < n; i++){
		d[i] = INFTY;
		p[i] = -1;
		color[i] = WHITE;
	}

	d[0] = 0;

	while (1){
		minv = INFTY;
		// u: node which has minv
		u = -1;
		for (long i = 0; i < n; i++){
			if (minv > d[i] && color[i] != BLACK){
				u = i;
				minv = d[i];
			}
		}
		// cut all nodes
		if (u == -1) break;
		color[u] = BLACK; // already selected
		for (long 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 (long i = 0; i < n; i++){
		if (p[i] != -1) sum += M[i][p[i]];
	}

	return sum;
}

int main(){
	cin >> n;

	for (long i = 0; i < n; i++){
		for (long j = 0; j < n; j++){
			long e;
			cin >> e;
			//M[i][j] = (e == -1) ? INFTY : e;
			if (e == -1) M[i][j] = INFTY;
			else M[i][j] = e;
		}
	}

	cout << prim() << endl;
}