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 <string>

using namespace std;


int P[10010]; // 0 ?????? 10000 ?????§????????????????????±?????????
void init(int N) { // ????????? ??????????????¨???????????????????????????
  for (int i=0; i<=N; ++i) P[i] = i;
  // ??¨: ????????? [0,N-1] ????????? [1,N] ?????????????????????
  // ???????????§???????????§???????????????????????§??? [0,N] ???????????¨??????
}
int root(int a) { // a ??? root(??£??¨???) ????±???????
  if (P[a] == a) return a; // a ??? root
  return (P[a] = root(P[a])); // a ????????? root ????±???????a ????????¨??????
}
bool is_same_set(int a, int b) { // a ??¨ b ???????????°?????????????±???????????
  return root(a) == root(b);
}
void unite(int a, int b) { // a ??¨ b ???????????°?????????????????¨??????
  P[root(a)] = root(b);
}


int N=0;
int A[100+10][100+10] = {0};

int cost[10100] = {0};
int src[10100] = {0};
int dst[10100] = {0};

pair<int,int> c_n[10100];



int main() {
  cin >> N;
  init(N+10);
  
  int k=0;
  for(int j=0;j<N;++j){
    for(int i=0;i<N;++i){
      cin >> A[i][j];
      //cout << A[i][j] << "|";
      if (A[i][j] != -1){
	cost[k] = A[i][j];
	src[k] = i;
	dst[k] = j;
	c_n[k] = make_pair(cost[k],k);
	++k;
      }
    }
    //cout << endl;
  }

  
  /*
  for (int i=0;i<20;++i){
    cout << c_n[i].first << " " << c_n[i].second << endl;
  }

  cout << endl << "SORT" << endl;

  
  for (int i=0;i<20;++i){
    cout << c_n[i].first << " " << c_n[i].second << endl;
  }*/

  sort(c_n, c_n+k);
  /*
  c_n[0].second
  ???????????????
  is_same_set
  ??????
  unite(src[],dst[])
?????????
  c_n[1].second????????????k???????????§????????°??????
  */

  int num = 0;
  int sum = 0;
  for(int i=0;i<k;++i){
    num = c_n[i].second;
    if(is_same_set(src[num], dst[num]) == 0){
      unite(src[num], dst[num]);
      sum += cost[num];
      //cout << num << " " << sum << endl;
    }
  }
  cout << sum << endl;
}

C++: #include <iostream>
using namespace std;
#define INF 1000000

int main(){
  int n;
  int G[100][100],d[100];
  int minc,mi,i;
  bool flg[i];
  int sum=0;
  
  cin >> n;
  for (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;
    }
    d[i]=INF;
    flg[i]=false;
  }

  d[0]=0;
  while (true){
    minc=INF;
    for (i=0;i<n;i++){
      if (!flg[i]&&d[i]<minc){
	minc=d[i];
	mi=i;
      }
    }
    if (minc==INF) break;
    flg[mi]=true;
    for (i=0;i<n;i++){
      if (G[mi][i]==INF)continue;
      if (!flg[i]&&d[i]>G[mi][i]){
	d[i]=G[mi][i];
	//cout << d[i];
      }
    }
  }
  
  for (i=0;i<n;i++){
    sum+=d[i];
  }
  cout << sum<<endl;
  
  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;
}

C++: #include<bits/stdc++.h>
using namespace std;

class union_find {
  vector<int> uf;

public:
  union_find(int sz)
  {
    uf = vector<int>(sz);
    for (int i = 0; i < sz; i++)
      uf[i] = i;
  }
  
  int root(int i)
  {
    if (uf[i] == i) return i;
    return uf[i] = root(uf[i]);
  }

  void unite(int i, int j)
  {
    int ri = root(i);
    int rj = root(j);
    if (ri != rj)
      uf[rj] = ri;
  }

  bool same(int i, int j)
  {
    return root(i) == root(j);
  }
};

typedef pair<int, int> edge;
typedef pair<edge, int> V;
priority_queue<V, vector<V>, function<bool(V, V)>> que(
  [](V a, V b) { return a.second >= b.second; }
);

#define make_V(i, j, w) make_pair(make_pair((i), (j)), (w))

int solve(int n)
{
  union_find uf(n);

  int ans = 0;
  while (!que.empty()){
    int i, j, w;
    V v = que.top(); que.pop();
    i = v.first.first, j = v.first.second;
    w = v.second;

    if (uf.same(i, j)) continue;
    uf.unite(i, j);
    ans += w;

    // printf("(%d, %d) %d\n", v.first.first, v.first.second, v.second);
  }

  return ans;
}

int main()
{
  int n; cin >> n;

  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      int weight; cin >> weight;
      if (j > i && weight != -1)
        que.push(make_V(i, j, weight));
    }
  }

  cout << solve(n) << endl;

  return EXIT_SUCCESS;
}