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<bits/stdc++.h>
using namespace std;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }

using Lint=long long;

class UnionFind
{
 public:
    vector<int> par,sz;
    UnionFind(int N):par(N),sz(N){
        for(int i=0;i<N;i++){
            par[i]=i; sz[i]=1;
        }
    }
    int find(int x){
        if(par[x]==x) return x;
        return par[x]=find(par[x]);
    }
    void unite(int x,int y){
        x=find(x); y=find(y);
        if(x==y) return;
        if(sz[x]<sz[y]) swap(x,y);
        par[y]=x;
        sz[x]+=sz[y];
    }
    bool same(int x,int y){ return find(x)==find(y); }
    int size(int x){ return sz[find(x)]; }
    bool root(int x){ return par[x]==x; }
};

struct edge{
    int u,v,cost;
    bool operator<(const edge &e){
        return cost<e.cost;
    }
};

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);

    int N; cin>>N;
    vector<edge> es;
    for(int i=0;i<N;i++){
        for(int j=0;j<N;j++){
            int c; cin>>c;
            if(i<j and c!=-1) es.push_back({i,j,c});
        }
    }
    sort(es.begin(),es.end());

    int ans=0;
    UnionFind tree(N);
    for(edge e:es){
        if(!tree.same(e.u,e.v)){
            ans+=e.cost;
            tree.unite(e.u,e.v);
        }
    }
    cout<<ans<<endl;
    return 0;
}

C++: #include<iostream>
#include<vector>
#include<utility>
#include<algorithm>
using namespace std;
class UnionFind{
  vector<int> node;
 public:
  UnionFind(int n):node(n){
    for(int i = 0;i < n;i++){
      node[i] = i;
    }
  }
  int root(int x){
    if(x == node[x])return x;
    return root(node[x]);
  }
  void merge(int x,int y){
    node[root(x)] = root(y);
  }
};
struct edge{
  int x,y,w;
};
bool operator<(edge e1,edge e2){
  return e1.w < e2.w;
}
int main(){
  int n;
  cin >> n;
  vector<edge> edges;
  UnionFind u(n);
  for(int i = 0;i < n;i++){
    for(int j = 0;j < n;j++){
      int x;
      cin >> x;
      if(x >= 0){
        edges.push_back({i,j,x});
      }
    }
  }
  int weight = 0;
  sort(edges.begin(),edges.end());
  for(auto &e:edges){
    if(u.root(e.x) == u.root(e.y))continue;
    u.merge(e.x,e.y);
    weight += e.w;
  }
  cout << weight << endl;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define MAX 100
#define WHITE 0
#define BLACK 1
#define INFTY 2001

// グラフの頂点数
int n;
// 隣接行列
vector<vector<int> > M(MAX, vector<int>(MAX));
// 頂点の状態を表す配列
vector<int> color(MAX, WHITE);
// d[v]はvとTの頂点を結ぶ辺の重みの最小値
vector<int> d(MAX, INFTY);
// p[v]はd[v]の重みを持つ辺のvでない方の頂点
vector<int> p(MAX, -1);

int mst()
{
  // 初期化
  int ans = 0;
  d.at(0) = 0;

  while (1)
  {
    // d[u]が最小であるようなV-Tの頂点uを選ぶ
    int u;
    int m = INFTY;
    for (int i = 0; i < n; i++)
    {
      if (color.at(i) != BLACK && d.at(i) < m)
      {
        m = d.at(i);
        u = i;
      }
    }

    if (m == INFTY)
    {
      // MNTが完成したとき
      break;
    }

    // uをTに追加する
    color.at(u) = BLACK;
    // uをTに追加したときの辺の重みをansに足す
    if (p.at(u) != -1)
    {
      ans += M.at(p.at(u)).at(u);
    }

    // uと隣接しているV-Tの各頂点iのd[i]を更新する
    for (int i = 0; i < n; i++)
    {
      if (color.at(i) != BLACK && M.at(u).at(i) != -1 && M.at(u).at(i) < d.at(i))
      {
        d.at(i) = M.at(u).at(i);
        p.at(i) = u;
      }
    }
  }

  return ans;
}

int main()
{
  // 入力
  cin >> n;
  for (int i = 0; i < n; i++)
  {
    for (int j = 0; j < n; j++)
    {
      cin >> M.at(i).at(j);
    }
  }

  // 処理と出力
  cout << mst() << endl;
}

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

int main(void)
{
	int n, now = 0, flag[101];
	long ans = 0, an[100][100], min[100];

	cin >> n;

	for(int i = 0; i < n; i++) {
		for(int j = 0; j < n; j++) {
			cin >> an[i][j];
		}
		min[i] = LONG_MAX;
		flag[i] = 0;
	}

	/* ???????則?????????即?????属????????? */
	do {
		flag[now] = 1;
		flag[100] = 0;
/*
		cout << "\n??? " << now << " ????則???????????????????" << endl;
		cout << "?????贈?????? : ";
		for(int i = 0; i < n; i++) {
			if(flag[i] == 1) {
				cout << i << " ";
			}
		}
		cout << endl;
		cout << "?????即???????属?????????則" << endl;
		for(int i = 0; i < n; i++) {
			if(flag[i] == 0) {
				cout << "??? " << i << " : " << min[i] << endl;
			}
		}
*/
		/* min?????卒??属 */
		for(int j = 0; j < n; j++) {
			if(an[now][j] != -1 && an[now][j] < min[j] && flag[j] == 0) {
/*				cout << "??? " << j <<" ???????属?????????卒??属??????????????? : "<< min[j] << " ??? " << an[now][j] << endl; */
				min[j] = an[now][j];
			}
		}
/*
		cout << "?????即???????属?????????則" << endl;
		for(int i = 0; i < n; i++) {
			if(flag[i] == 0) {
				cout << "??? " << i << " : " << min[i] << endl;
			}
		}
*/
		/* ?測??????蔵?卒蔵???min????属???? */
		long key = LONG_MAX, key_j;

		for(int j = 0; j < n; j++) {
			if(flag[j] == 0 && min[j] < key) {
				key = min[j];
				key_j = j;
			}
		}
/*
		cout << "?則???????????賊??????????????????? : ??? " << key_j << endl;
*/
		/* ?則?????????? */
		now = key_j;
		ans += key;

		/* ??????????????贈????????? */
		for(int i = 0; i < n; i++) {
			if(flag[i] == 1) {
				flag[100]++;
			}
		}
	
	}while(flag[100] != n - 1);

	cout << ans << endl;

	return 0;
}