Go: package main

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

const (
	MAX   = 100
	INFTY = 1 << 21
	NIL   = -1
)

const (
	WHITE = iota
	GRAY
	BLACK
)

type Graph [MAX][MAX]int

func (g *Graph) prim(n int) int {
	var (
		u, minv     = 0, 0
		d, p, color [MAX]int
	)

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

	for {
		minv = INFTY
		u = -1
		for 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 v := 0; v < n; v++ {
			if color[v] != BLACK && g[u][v] != INFTY {
				if d[v] > g[u][v] {
					d[v] = g[u][v]
					p[v] = u
					color[v] = GRAY
				}
			}
		}
	}

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

	return sum
}

func answer(reader io.Reader) string {
	sc := bufio.NewScanner(reader)
	sc.Split(bufio.ScanWords)

	sc.Scan()
	n, _ := strconv.Atoi(sc.Text())

	var g Graph
	for i := 0; i < n; i++ {
		for j := 0; j < n; j++ {
			sc.Scan()
			e, _ := strconv.Atoi(sc.Text())
			if e == -1 {
				g[i][j] = INFTY
				continue
			}
			g[i][j] = e
		}
	}

	return fmt.Sprint(g.prim(n))
}

func main() {
	fmt.Println(answer(os.Stdin))
}

C++: #include <utility>
#include <string>
#include <vector>
#include <algorithm>
#include <iostream>
using namespace std;

int n;					//えぬ
int a[100][100] = {0};	//辺の重さ
int f[100] = {0};		//その頂点がすでに連結済みかどうか、その集団の番号
int c = 0;				//全体の重さ
int d = 1;				//次につける集団番号
int s = 0;				//選んだ本数
vector<int> w;			//木のリスト、軽いもの順
int wc = 0;				//見てる木のリストの位置

int main(){
	cin >> n;
	//まず辺の重さをaに入れる
	//wに辺のリストを入れる
	for ( int i = 0; i < n; i++ ) {
		for ( int j = 0; j < n; j++ ) {
			cin >> a[i][j];
			if ( a[i][j] != -1 && i < j ) w.push_back( a[i][j] );
		}
	}
	sort( w.begin(),w.end() );
	while(true) {									//n-1本選ぶまで無限ループ
		for ( int i = 0; i < n - 1; i++ ) {			// i < j を満たすように木を調べ続ける
			for ( int j = i + 1; j < n; j++ ) {
				if ( a[i][j] != w[wc] ) continue;	//その木が一番軽い木だったらそれを対象に検査を始める
				if ( f[i] > 0 && f[j] > 0 ) {		//その木の両端が両方とも連結済みだったら
					if ( f[i] != f[j] ) {
						int y = f[j];
						for ( int x = 0; x < n; x++ ) {
							if ( f[x] == y ) f[x] = f[i];
						}
						c += a[i][j];
						s++;
						cout << i << " " << j << endl;
					}
				}else if ( f[i] == 0 && f[j] == 0 ) {
					f[i] = d;
					f[j] = d;
					d++;
					c += a[i][j];
					s++;
					cout << i << " " << j << endl;
				}else {
					if ( f[i] == 0 ) {
						f[i] = f[j];
						c += a[i][j];
						s++;
					}else {
						f[j] = f[i];
						c += a[i][j];
						s++;
					}
					cout << i << " " << j << endl;
				}
				wc++;
				a[i][j] = -1;
				if ( s == n-1 ) break;
			}
			if ( s == n-1 ) break;
		}
		if ( s == n-1 ) break;
	}
	cout << c << endl;
	return 0;
}

C++: #include<iostream>
#include<vector>
#include<algorithm>
#include<cstdio>
#include<cstdlib>
#include<string>
#include<sstream>
#include<cmath>
#include<numeric>
#include<map>
#include<stack>
#include<queue>
using namespace std;
int inf = 1000000000;

class UF{
public:
  vector<int> u;
  vector<int> r;

  void init(int n){
    u.resize(n, 0);
    r.resize(n, 0);
    for(int i=0; i<n; i++) u[i] = i;
  }
  int root(int x){
    // if( u[x] == x ) return x;
    // else return root(u[x]);
    return u[x] == x ? x : u[x] = root(u[x]);
  }
  bool same(int x, int y){
    return root(x) == root(y);
  }
  void unite(int x, int y){
    x = root(x);
    y = root(y);
    if( x == y ) return;

    if( r[x] < r[y] ){
      u[x] = y;
    }else{
      u[y] = x;
      if( r[x] == r[y] ) r[x]++;
    }
  }
};
class Graph{
public:
  int from, to, cost;
  bool operator<(const Graph &g1)const{
    return g1.cost < cost;
  };
};

int main(void) {

  int n;
  cin >> n;
  UF uf;
  uf.init(n);
  priority_queue<Graph> g;
  for(int i=0; i<n; i++){
    for(int j=0; j<n; j++){
      int a;
      cin >> a;
      if( a == -1 ) continue;
      Graph tg;
      tg.to = i;
      tg.from = j;
      tg.cost = a;
      g.push(tg);
    }
  }

  int sum = 0;
  while( !g.empty() ){
    Graph p = g.top();
    g.pop();

    if( uf.same(p.to, p.from) ) continue;

    sum += p.cost;
    uf.unite(p.to, p.from);

  }
  cout << sum << endl;

  return 0;
}

// EOF

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <functional>

using namespace std;
typedef pair<int, int> P;   //cost, to
static const int INF = 1e9;
static const int SIZE = 101;
int par[SIZE];
int rk[SIZE];
int d[SIZE];
int n;

struct edge
{
	int u, v, cost;
};

struct edge2
{
	int to, cost;
};

bool comp(const edge& e1, const edge& e2)
{
	return(e1.cost < e2.cost);
}

void init()
{
	for (int i = 0; i < SIZE; ++i)
	{
		par[i] = i;
		rk[i] = 0;
	}
}

int find(int x)
{
	if (x == par[x])
	{
		return(x);
	}
	else
	{
		return(par[x] = find(par[x]));
	}
}

void unite(int a, int b)
{
	int x = find(a);
	int y = find(b);
	if (x == y)
	{
		return;
	}
	if (rk[x] < rk[y])
	{
		par[x] = y;
	}
	else
	{
		par[y] = x;
		if (rk[x] == rk[y])
		{
			++rk[x];
		}
	}
}

bool same(int a, int b)
{
	return(find(a) == find(b));
}

int kruskal(vector<edge>& es)
{
	init();
	int res = 0;
	sort(es.begin(), es.end(), comp);
	for (int i = 0; i < es.size(); ++i)
	{
		edge e = es[i];
		if (!same(e.u, e.v))
		{
			unite(e.u, e.v);
			res += e.cost;
		}
	}
	return(res);
}

int prim(vector<edge2>* G)
{
	vector<bool> isUsed(n);
	for (int i = 0; i < n; ++i)
	{
		isUsed[i] = false;
	}
	priority_queue<P, vector<P>, greater<P> > pque;
	P p = make_pair(0, 0);
	pque.push(p);
	int res = 0;
	while (!pque.empty())
	{
		P p = pque.top();
		pque.pop();
		int v = p.second;
		if (isUsed[v])
		{
			continue;
		}
		isUsed[v] = true;
		res += p.first;
		for (int i = 0; i < G[v].size(); ++i)
		{
			edge2 e = G[v][i];
			pque.push(P(e.cost, e.to));
		}
	}
	return(res);
}


void solve()
{
	cin >> n;

// For Kruskal
	vector<edge> es;
	for (int i = 0; i < n; ++i)
	{
		for (int j = 0; j < n; ++j)
		{
			int cost;
			cin >> cost;
			if (cost != -1)
			{
				edge e;
				e.u = i;
				e.v = j;
				e.cost = cost;
				es.push_back(e);
			}
		}
	}
	cout << kruskal(es) << endl;

// For Prim
	//vector<edge2> G[SIZE];
	//for (int i = 0; i < n; ++i)
	//{
	//	for (int j = 0; j < n; ++j)
	//	{
	//		int cost;
	//		cin >> cost;
	//		if (j >= i && cost != -1)
	//		{
	//			edge2 e1, e2;
	//			e1.to = j;
	//			e1.cost = cost;
	//			e2.to = i;
	//			e2.cost = cost;
	//			G[i].push_back(e1);
	//			G[j].push_back(e2);
	//		}
	//	}
	//}
	//cout << prim(G) << endl;
}

int main()
{
	solve();
	return(0);
}