Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

type ElementKey int
type Node struct {
	key    ElementKey
	parent *Node
	rank   int
}

func newNode(key ElementKey) *Node {
	node := Node{}
	node.key = key
	node.parent = &node
	node.rank = 0

	return &node
}

func union(x, y *Node) {
	xSet := findSet(x)
	ySet := findSet(y)
	link(xSet, ySet)
}

func link(x, y *Node) {
	if x.rank > y.rank {
		y.parent = x
	} else {
		x.parent = y

		if x.rank == y.rank {
			y.rank++
		}
	}
}

func findSet(x *Node) *Node {
	if x != x.parent {
		x.parent = findSet(x.parent)
	}
	return x.parent
}

func newDisjointSets(nSets int) *DisjointSets {
	sets := DisjointSets{}
	sets.keyToNode = map[ElementKey]*Node{}
	for i := 0; i < nSets; i++ {
		key := ElementKey(i)
		sets.keyToNode[key] = newNode(key)
	}
	return &sets
}

type DisjointSets struct {
	keyToNode map[ElementKey]*Node
}

func (sets *DisjointSets) union(x, y ElementKey) {
	xNode := sets.keyToNode[x]
	yNode := sets.keyToNode[y]
	union(xNode, yNode)
}

func (sets DisjointSets) findSet(x ElementKey) *Node {
	xNode := sets.keyToNode[x]
	return findSet(xNode)
}

type Edge struct {
	i, j   int
	weight int
}

func main() {
	scanner.Split(bufio.ScanWords)

	n := nextInt()
	sets := newDisjointSets(n)
	edges := []Edge{}
	a := make([][]int, n)
	for i := 0; i < n; i++ {
		a[i] = make([]int, n)
		for j := 0; j < n; j++ {
			a[i][j] = nextInt()
			if a[i][j] >= 0 {
				e := Edge{
					i:      i,
					j:      j,
					weight: a[i][j],
				}
				edges = append(edges, e)
			}
		}
	}
	sort.Slice(edges, func(i, j int) bool { return edges[i].weight < edges[j].weight })
	totalWeight := 0
	for _, edge := range edges {
		i := ElementKey(edge.i)
		j := ElementKey(edge.j)
		if sets.findSet(i) != sets.findSet(j) {
			totalWeight += edge.weight
			sets.union(i, j)
		}
	}
	fmt.Println(totalWeight)
}

C++: // Graph II - Minimum Spanning Tree (AOJ)                         
// http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ALDS1_ 12_A&lang=jp
// https://onlinejudge.u-aizu.ac.jp/#/problems/ALDS1_12_A
// 中央×前

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int P[10000];

// n個の頂点を初期化する
void init(int n) {
	for (int i = 0; i < n; i++) P[i] = i;
}

// 頂点の親の番号を返す
int root(int a) {
	if (P[a] == a) return a;
	return (P[a] = root(P[a]));
}

// 親(集合)が同じか判定する
bool is_same_set(int a, int b) {
	return root(a) == root(b);
}

// aとbを同一グループにまとめる
void unite(int a, int b) {
	P[root(a)] = root(b);
}

int main(void) {
	int n;
	cin >> n;
	vector<vector<int>> A(n, vector<int>(n, 0));
	vector<vector<int>> v;
	vector<int> w(3, 0);
	int a;
	for(int i = 0; i < n; i++) {
		for(int j = 0; j < n; j++) {
			cin >> a;
			w[0] = a;
			w[1] = i;
			w[2] = j;
			v.push_back(w);
		}
	}

	sort(v.begin(), v.end());

	init(n);
	int sum = 0;

	for(vector<int> i : v) {
	//	cout << "("<< i[0] << "," << i[1] << "," << i[2] << ")" << endl;
		if (i[0] == -1) continue;
		if (is_same_set(i[1], i[2])) continue;
	//	cout << i[0] << "を追加！！" << endl;
		sum += i[0];
		unite(i[1], i[2]); 
	//	for(int j = 0; j < n; j++) cout << j << ":" << P[j] << "/";
	}
	cout << sum << endl;
}

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<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 <stdio.h>
#include <queue>
#include <vector>
#include <functional>

#define N 100
using namespace std;


class list {
public:
	int cost;
	int node;
	list(void) {
		cost = -1;
		node = -1;
	}
};

bool operator<(const list& y,const list& x) {
	return (y.cost < x.cost);
};
bool operator>(const list& y,const list& x) {
	return (y.cost>x.cost);
};

int m[N][N];
bool visited[N];
int sum = 0;

int main(void) {
	priority_queue<list, vector<list>, greater<list> > pq;
	int n;
	int now;
	scanf("%d", &n);
	for (int i = 0; i<n; i++) {
		visited[i] = false;
		for (int j = 0; j<n; j++) {
			scanf("%d", &m[i][j]);
		}
	}
	now = 0;
	while (1) {
		//printf("I'm in %d\n", now);
		visited[now] = true;
		for (int j = 0; j<n; j++) {
			if (m[now][j] != -1) {
				list a;
				a.cost = m[now][j];
				a.node = j;
				pq.push(a);
			//	printf("I pushed %d %d\n", a.node, a.cost);
			}
		}
		//system("pause");
		list x;
		while (!pq.empty()) {
			if (visited[(pq.top()).node] == false) {
				x = pq.top();
				//printf("I got %d %d\n", x.node, x.cost);
				pq.pop();
				now = x.node;
				sum += x.cost;
				break;
			}
			else pq.pop();
		}
		if (x.node == -1)break;
	}
	printf("%d\n", sum);
	return 0;
}