Go: package main

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

//util
var sc = bufio.NewScanner(os.Stdin)

func nextStr() string {
	sc.Scan()
	return sc.Text()
}

func nextInt() int {
	sc.Scan()
	i, e := strconv.Atoi(sc.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func printList(list []int) {
	n := len(list)
	buf := bufio.NewWriter(os.Stdout)
	for _, v := range list[0 : n-1] {
		buf.Write([]byte(fmt.Sprintf("%d ", v)))
	}
	buf.Write([]byte(fmt.Sprintf("%d\n", list[n-1])))
	buf.Flush()
}

func max(a, b int) int {
	if a > b {
		return a
	} else {
		return b
	}
}

func min(a, b int) int {
	if a > b {
		return b
	} else {
		return a
	}
}

//solve
const (
	MAX   = 100
	INFTY = (1 << 21)
	WHITE = 0
	GRAY  = 1
	BLACK = 2
)

//V: グラフの頂点全体の集合
//T: MSTに属する頂点の集合
var (
	n     int
	M     [MAX][MAX]int //重みを記録する隣接行列
	color [MAX]int
	p     [MAX]int //MSTにおける頂点の親を記録
	d     [MAX]int //Tに属する頂点と、V−Tに属する頂点を繋ぐ辺の中で、重みが最小のものを記録
)

func prim() int {
	var u, minv int
	//初期化
	for i := 0; i < n; i++ {
		d[i] = INFTY
		p[i] = -1
		color[i] = WHITE
	}

	d[0] = 0

	for {
		//頂点uを決めていく
		//つまり、Tに属する頂点と、V−Tに属する頂点を繋ぐ辺の中で、重みが最小のものを決める
		minv = INFTY
		u = -1
		for i := 0; i < n; i++ {
			//未訪問かつ、辺の重みが最小となるuを選ぶ
			if minv > d[i] && color[i] != BLACK {
				u = i
				minv = d[i]
			}
		}

		if u == -1 {
			break
		}

		color[u] = BLACK
		for v := 0; v < n; v++ {
			//未訪問かつ、辺が存在するとき
			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
				}
			}
		}
	}

	sum := 0
	for i := 0; i < n; i++ {
		if p[i] != -1 {
			sum += M[i][p[i]]
		}
	}
	return sum
}

func main() {
	sc.Split(bufio.ScanWords)
	n = nextInt()

	for i := 0; i < n; i++ {

		for j := 0; j < n; j++ {
			tmp := nextInt()
			if tmp == -1 {
				tmp = INFTY
			}
			M[i][j] = tmp
		}

	}

	fmt.Println(prim())

}

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

vector<int> P;
int n;

void init() {
  for (int i = 0; i < n; i++)
    P[i] = i;
}
int root(int c) {
  if (P[c] == c)
    return c;
  return (P[c] = root(P[c]));
}
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() {
  cin >> n;
  P = vector<int>(n);
  init();
  vector<tuple<int, int, int>> edges; //<weight, start, end>
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      int a;
      cin >> a;
      if (i > j && a != -1)
        edges.push_back(make_tuple(a, i, j));
    }
  }
  sort(edges.begin(), edges.end());

  int ans = 0;
  for (tuple<int, int, int> edge : edges) {
    int w = get<0>(edge), s = get<1>(edge), e = get<2>(edge);
    if (!is_same_set(s, e)) {
      unite(s, e);
      ans += w;
    }
  }
  cout << ans << endl;
}

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

class UnionFind{
    int N;
    vector<int> par;
    vector<int> rank;
    public:
    UnionFind(int N):N(N),par(N),rank(N,0){iota(par.begin(),par.end(),0);}
    int find(int x);
    void unite(int x,int y);
    bool same(int x,int y){return find(x)==find(y);}
};

int UnionFind::find(int x){
    if(par[x]==x) return x;
    return par[x]=find(par[x]);
}
void UnionFind::unite(int x,int y){
    x=find(x),y=find(y);
    if(x==y) return;
    if(rank[x]>=rank[y]){
        if(rank[x]==rank[y]) rank[x]++;
        par[y]=x;
    }else{
        par[x]=y;
    }
}

using Weight=long long;
struct Edge{
    int from,to;
    Weight weight;
};
bool operator<(const Edge& lhs,const Edge& rhs){
    return lhs.weight<rhs.weight;
}

using Edges=vector<Edge>;
using Graph=vector<Edges>;


Edges MinimumSpanningTree(int n,Edges edges){
    Edges res;
    UnionFind u(n);
    for(int i=0;i<edges.size();i++){
        if(!u.same(edges[i].from,edges[i].to)){
            res.push_back(edges[i]);
            u.unite(edges[i].from,edges[i].to);
        }
    }
    return res;
}

Edges MinimumSpanningTree(const Graph& g){
    Edges edges;
    for(int i=0;i<g.size();i++){
        copy(g[i].begin(),g[i].end(),back_inserter(edges));
    }
    auto res=MinimumSpanningTree(g.size(),edges);
    return res;
}

int main(){
    int n;
    cin>>n;
    Graph g(n);
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            Weight c;
            cin>>c;
            if(i==j) continue;
            g[i].push_back({i,j,c});
        }
    }
    auto mst=MinimumSpanningTree(g);
    cout<<accumulate(mst.begin(),mst.end(),Weight(0),[](const Weight& lhs,const Edge& rhs){return lhs+rhs.weight;})<<endl;
    return 0;
}

C++: #include <algorithm>
#include <iostream>
#include <vector>
#include <math.h>
#include <set>
#include <map>
#include <string>
#include <stack>
#include <queue>
#include <iomanip>
#include <numeric>
#define _USE_MATH_DEFINES
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
double pi = 3.141592653589793;
ll mod = 1000000007;
int intmax = 2147483647;
int intmin = -2147483648;
ll llmax = 9223372036854775807;
ll llmin = -9223372036854775808;

struct unionFind {
    vector<int> data;
    unionFind(int size) : data(size, -1) {}
    
    bool unite(int x, int y) {
        x = root(x);
        y = root(y);
        if (x != y) {
            if(data[y] < data[x]) {
                swap(x, y);
            }
            data[x] += data[y];
            data[y] = x;
        }
        return x != y;
    }

    bool same(int x, int y) {
        return root(x) == root(y);
    }

    int root(int x) {
        return (data[x] < 0) ? x : data[x] = root(data[x]);
    }

    int rank(int x) {
        return -data[root(x)];
    }
};

struct edge {
    int from, to, cost;
};

bool asc (const edge &e, const edge &f) {
    return (e.cost != f.cost) ? e.cost < f.cost : (e.from != f.from ? e.from < f.from : e.to < f.to);
}

int mat[200][200];

int main() {
    int N;
    cin >> N;
    vector<edge> G;
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            cin >> mat[i][j];
        }
    }
    for (int i = 0; i < N; i++) {
        for (int j = i + 1; j < N; j++) {
            if (mat[i][j] != -1) {
                edge e;
                e.from = i;
                e.to = j;
                e.cost = mat[i][j];
                G.push_back(e);
            }
        }
    }
    sort(G.begin(), G.end(), asc);
    int ans = 0;
    int num = 0;
    unionFind u(N);
    for (int i = 0; i < G.size(); i++) {
        if (u.root(G[i].from) != u.root(G[i].to)) {
            u.unite(G[i].from, G[i].to);
            ans += G[i].cost;
            num++;
        }
        if (num == N - 1) {
            break;
        }
    }
    cout << ans << endl;
}

C++: /*
* @Author: Samson
* @Date:   2018-06-12 18:24:44
* @Last Modified by:   Samson
* @Last Modified time: 2018-06-13 12:20:56
*/
//   @URL : http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ALDS1_12_A
#include<bits/stdc++.h>
#include<algorithm>
#include <cstdlib>
#define INF 0x3f3f3f3f
using namespace std;
typedef long long LL;
const int MAXN = 1e2+10;

int p[MAXN],color[MAXN],a[MAXN][MAXN],t,n,d[MAXN];

int prim()
{
	int minv,u,v;
	memset(p,-1,sizeof p);
	memset(d,INF,sizeof d);

	d[1] = 0;

	while(1)
	{
		minv = INF;
		u = -1;
		for(int i = 1; i <= n; ++i)
		{
			if(minv > d[i] && !color[i])
				minv = d[i],u = i;
		}
		if(u == -1)	break;
		color[u] = 1;
		for(int v = 1; v <= n; ++v)
		{
			if(!color[v] && a[u][v] != INF)
			{
				if(d[v] > a[u][v])
				{
					d[v] = a[u][v];
					p[v] = u;
				}
			}
		}
	}
	
	int ans = 0;
	for(int i = 1; i <= n; ++i)
		if(p[i] != -1)
			ans += a[i][p[i]];
	return ans;
}

int main(void)
{
	ios::sync_with_stdio(false); 
	//cin.tie(0);
	cin>>n;
	for(int i = 1; i <= n; ++i)
	{
		for(int j = 1; j <= n; ++j)
		{
			cin>>t;
			a[i][j] = t==-1?INF:t;
		}
	}
	cout<<prim()<<'\n';
	return 0;
}