Go: package main

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

// I/O
type Scanner struct {
	sc *bufio.Scanner
}

func NewScanner() *Scanner {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	sc.Buffer(make([]byte, 1024), int(1e+9))
	return &Scanner{sc}
}

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

func (s *Scanner) nextInt() int {
	i, e := strconv.Atoi(s.nextStr())
	if e != nil {
		panic(e)
	}
	return i
}

func (s *Scanner) nextFloat() float64 {
	f, e := strconv.ParseFloat(s.nextStr(), 64)
	if e != nil {
		panic(e)
	}
	return f
}

func (s *Scanner) nextRuneSlice() []rune {
	return []rune(s.nextStr())
}

func (s *Scanner) nextIntSlice(n int) []int {
	res := make([]int, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextInt()
	}
	return res
}

func (s *Scanner) nextFloatSlice(n int) []float64 {
	res := make([]float64, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextFloat()
	}
	return res
}

// Arithmetic
func max(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m < i {
			m = i
		}
	}
	return m
}

func min(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m > i {
			m = i
		}
	}
	return m
}

func abs(x int) int {
	if x > 0 {
		return x
	}
	return -x
}

func pow(x, y int) int {
	res := 1
	for i := 0; i < y; i++ {
		res *= x
	}
	return res
}

func ceil(a, b int) int {
	if a%b == 0 {
		return a / b
	} else {
		return a/b + 1
	}
}

// Sort
type RuneSlice []rune

func (a RuneSlice) Len() int           { return len(a) }
func (a RuneSlice) Less(i, j int) bool { return a[i] < a[j] }
func (a RuneSlice) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }

const INF = 1 << 60

// Main
func main() {
	sc := NewScanner()
	wtr := bufio.NewWriter(os.Stdout)
	N := sc.nextInt()
	G := make([][]int, N)
	for i := 0; i < N; i++ {
		G[i] = make([]int, N)
		for j := 0; j < N; j++ {
			G[i][j] = sc.nextInt()
		}
	}
	d := make([]int, N)
	for i := 0; i < N; i++ {
		d[i] = INF
	}
	seen := make([]bool, N)
	d[0] = 0
	ans := 0
	for {
		cur := 0
		minCost := INF
		for i := 0; i < N; i++ {
			if !seen[i] && d[i] < minCost {
				minCost = d[i]
				cur = i
			}
		}
		if minCost == INF {
			break
		}
		seen[cur] = true
		ans += d[cur]
		for i := 0; i < N; i++ {
			cost := G[cur][i]
			if cost != -1 && cost < d[i] {
				d[i] = cost
			}
		}
	}

	fmt.Fprintln(wtr, ans)
	wtr.Flush()
}

C++: #include <algorithm>
#include <iostream>
#include <set>
#include <vector>

class TEdge;
struct TNode
{
	int TreeIndex;
	std::set<const TEdge*> Connected;
	
	TNode();
	void SetIndex(int Index, const TEdge* Edge);
};

class TEdge
{
	std::pair<TNode*, TNode*> FEnds;
	int FWeight;
public:
	TEdge(TNode* EndA, TNode* EndB, int Weight);
	
	bool operator<(const TEdge& Other)const;
	int  Weight()const {return FWeight;}
	bool ToMekeCycle()const;
	
	void JoinTree(int TreeIndex)const;
};

TNode::TNode()
:TreeIndex(0)
{
}

void TNode::SetIndex(int Index, const TEdge* Edge)
{
	if(TreeIndex >= Index) return;
	TreeIndex = Index;
	for(std::set<const TEdge*>::const_iterator It = Connected.begin(); It != Connected.end(); ++It){
		(*It)->JoinTree(Index);
	}
	Connected.insert(Edge);
}

TEdge::TEdge(TNode* EndA, TNode* EndB, int Weight)
:FEnds(EndA, EndB), FWeight(Weight)
{
}

bool TEdge::operator<(const TEdge& Other)const
{
	return Weight() < Other.Weight();
}

bool TEdge::ToMekeCycle()const
{
	return FEnds.first->TreeIndex > 0 && FEnds.first->TreeIndex == FEnds.second->TreeIndex;
}

void TEdge::JoinTree(int TreeIndex)const
{
	FEnds.first->SetIndex(TreeIndex, this);
	FEnds.second->SetIndex(TreeIndex, this);
}

class TGraph
{
	std::vector<TNode> FNodes;
	std::vector<TEdge> FEdges;
public:
	TGraph(int NodeCount);
	
	void AddEdge(int NodeIndexA, int NodeIndexB, int Weight);
	int WeightMinimumSpanningTree();
};

TGraph::TGraph(int NodeCount)
:FNodes(NodeCount)
{
}

void TGraph::AddEdge(int NodeIndexA, int NodeIndexB, int Weight)
{
	if(NodeIndexA <= NodeIndexB || Weight < 0) return;
	FEdges.push_back(TEdge(&FNodes[NodeIndexA], &FNodes[NodeIndexB], Weight));
}

int TGraph::WeightMinimumSpanningTree()
{
	std::sort(FEdges.begin(), FEdges.end());
	unsigned int TreeCount = 0;
	int Weight = 0;
	for(std::vector<TEdge>::iterator It = FEdges.begin(); It != FEdges.end() && TreeCount < FNodes.size() - 1; ++It){
		if(It->ToMekeCycle()) continue;
		It->JoinTree(TreeCount + 1);
		Weight += It->Weight();
		++TreeCount;
	}
	return Weight;
}

int main()
{
	int NodeCount;
	std::cin >> NodeCount;
	TGraph Graph(NodeCount);
	
	for(int i = 0; i < NodeCount; ++i)
	{
		for(int j = 0; j < NodeCount; ++j){
			int Weight;
			std::cin >> Weight;
			Graph.AddEdge(i, j, Weight);
		}
	}
	
	std::cout << Graph.WeightMinimumSpanningTree() << std::endl;
	return 0;
}

C++: // Graph II - Minimum Spanning Tree
// 問題URL: http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ALDS1_12_A&lang=jp
//
// 使うライブラリも多く、難しく感じた...
#include <iostream>
#include <algorithm>

using namespace std;

int n, buff, cost[101][101]={}, P[101], M = 0, ans = 0;
pair<int, int> edges[10010], vertices[10010];

void init(int n){
  for(int i = 0; i < n; ++i){
    P[i] = i;
  }
}

int root(int x){
  if(P[x]==x) return x;
  P[x] = root(P[x]);
  return P[x];
}

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

void unite(int x, int y){
  P[root(x)] = root(y);
}

int main(){
  cin >> n;

  for(int row = 0; row < n; ++row){
    for(int col = 0; col < n; ++col){
      cin >> buff;
      cost[row][col] = buff; //隣接行列の読込
    }
  }

  for(int row = 0; row < n; ++row){
    for(int col = 0; col < row; ++col){
      if(cost[row][col]>0){
        edges[M].first = cost[row][col];
        edges[M].second = M;
        //cout << M << ": (" << row << ", " << col << ") = " << edges[M].first << endl;
        vertices[M].first = row, vertices[M].second = col;
        M++; //辺の数を管理、idとして利用
      }
    }
  }

  init(n);

  sort(edges, edges+M); //重みの小さい順に整列

  for(int i = 0; i <= M; ++i){
    int index = edges[i].second; //辺のidを取り出す
    if(!is_same_set(vertices[index].first, vertices[index].second)){ //辺の両端が繋がっていなければ
      unite(vertices[index].first, vertices[index].second); //両端を繋げる
      cout << "unite " << vertices[index].first << " and " << vertices[index].second << endl;
      ans += edges[i].first; //コストを加える
    }
  }
  cout << ans << endl;
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define FOR(I,A,B) for(int I = (A); I < (B); ++I)
#define CLR(mat) memset(mat, -1, sizeof(mat))
#define N 101
struct edge{
	int to,cost;
};
int par[N];
int a,n,ans=0;
priority_queue< pair<int,pair<int,int> > > Q;

void init(int n){
	FOR(i,1,n+1)par[i]=i;
}

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

void unite(int x,int y){
	x=root(x);
	y=root(y);
	if(x==y)return;
	par[x]=y;
}


int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	cin>>n;
	init(n);
	FOR(i,1,n+1){
		FOR(j,1,n+1){
			cin>>a;
			if(a==(-1))a=2001;
			Q.push(make_pair(-a,make_pair(i,j)));
		}
	}
	while(Q.size()){
		pair<int,int> e;
		int cost,x,y;
		pair<int,pair<int,int> > q;
		q=Q.top();
		e=q.second;
		cost=-q.first;
		x=e.first;
		y=e.second;
		Q.pop();
		if(root(x)==root(y))continue;
		unite(x,y);
		ans+=cost;
	}
	cout<<ans<<endl;
	return 0;
}