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++: //============================================================================
// Name        : MinimumSpanningTree.cpp
// Author      : Ken
// Version     :
// Copyright   : Your copyright notice
// Description : Hello World in C++, Ansi-style
//============================================================================

#include <iostream>
#include <vector>
#include <set>
using namespace std;

struct Edge;

struct Node{
	const int id;
	vector<int> adjEdgeIds;
};

struct Edge{
	int id;
	int weight;
	int startNodeId;
	int endNodeId;
};

class Graph{
public:
	int MAX_WEIGHT=2000;
	vector<Node> nodeList;
	vector<Edge> edgeList;

	Graph():nodeList(),edgeList(){}

	Graph(int n):nodeList(){
		for(int i=0; i<n; i++){
			vector<int> empty;
			Node node = {i, empty};
			nodeList.push_back(node);
		}
		//cout << "initialized\n";
	}

	void write(){
		cout << "***nodeList***\n";
		for(int i=0; i<nodeList.size(); i++){
			Node& node = nodeList.at(i);
			cout << "node id: " << node.id << " ";
			for(int j=0; j<node.adjEdgeIds.size(); j++){
				Edge& edge = edgeList.at(node.adjEdgeIds.at(j));
				cout << "edge id: " << edge.id << " ";
				cout << edge.startNodeId << "<->" << edge.endNodeId << " ";
			}
			cout << "\n";
		}

		cout << "***edgeList***\n";
		for(int i=0; i<edgeList.size(); i++){
			Edge& edge = edgeList.at(i);
			cout << "edge id: " << edge.id << " ";
			cout << "edge weight: " << edge.weight << " ";
			cout << edge.startNodeId << "<->" << edge.endNodeId << "\n";
		}
	}

	void addLightestEdge(set<int>& edgeIds){
		set<int> curNodeIds;
		if(edgeIds.size()==0){
			curNodeIds.insert(0);
		}else{
			set<int>::iterator p;
			for(p=edgeIds.begin(); p!=edgeIds.end(); p++){
				Edge& edge = edgeList.at(*p);
				curNodeIds.insert(edge.startNodeId);
				curNodeIds.insert(edge.endNodeId);
			}
		}

		while(true){
		set<int>::iterator p;
		int lightestWeight = MAX_WEIGHT;
		int lightestEdgeId = -1;
		for(p=curNodeIds.begin(); p!=curNodeIds.end(); p++){
			Node& node = nodeList.at(*p);

			vector<int>::iterator q;
			for(q=node.adjEdgeIds.begin(); q!=node.adjEdgeIds.end(); q++){

				if(curNodeIds.count(edgeList.at(*q).startNodeId) == 0 or
				   curNodeIds.count(edgeList.at(*q).endNodeId) == 0){

					if(edgeList.at(*q).weight < lightestWeight){
						lightestWeight = edgeList.at(*q).weight;
						lightestEdgeId = edgeList.at(*q).id;
					}
				}
			}
		}

		if(lightestEdgeId!=-1){
		    edgeIds.insert(lightestEdgeId);
		    //cout << "inserted edge id: " << lightestEdgeId << "\n";
		    //cout << "inserted edge weight: " << lightestWeight << "\n";
		    Edge& edge = edgeList.at(lightestEdgeId);
		    curNodeIds.insert(edge.startNodeId);
		    curNodeIds.insert(edge.endNodeId);
		}else{
			return;
		}
		}
	}

	int sumEdgeWeight(set<int> edgeIdSet){
		int sum = 0;
		set<int>::iterator p;
		for(p=edgeIdSet.begin(); p!=edgeIdSet.end(); p++){
			sum += edgeList.at(*p).weight;
		}

		return sum;
	}
};

void writeSet(set<int> intSet){
	set<int>::iterator p;
	for(p=intSet.begin(); p!=intSet.end(); p++){
		cout << *p << " ";
	}
}



int main() {
	int n;
	cin >> n;
	Graph g(n);

	//g.write();

	for(int i=0; i<n; i++){
		for(int j=0; j<n; j++){
			int weight;
			cin >> weight;
			if(i<j and weight != -1){
				int edgeId = g.edgeList.size();
				Edge edge = {edgeId, weight, i, j};
				g.edgeList.push_back(edge);
				g.nodeList.at(i).adjEdgeIds.push_back(edgeId);
				g.nodeList.at(j).adjEdgeIds.push_back(edgeId);
			}
		}
	}

	//g.write();

	set<int> edgeIds;
	g.addLightestEdge(edgeIds);
	//writeSet(edgeIds);
	int totalWeight;
	totalWeight = g.sumEdgeWeight(edgeIds);
	cout << totalWeight << "\n";

	return 0;
}

C++: #include <iostream>
#include <queue>

using namespace std;




struct link { //link connecting x and y vertices
  int x;
  int y;
  int weight;
  bool operator<(const link& other) const
  {
    return weight > other.weight; // for min priority queue, trick the operator
  }
};

priority_queue<link> links;
int roots[100];

int root(int x) {
  if (x == roots[x]) {
    return x;
  } else {
    roots[x] = root(roots[x]); //attach all to root
    return root(roots[x]);
  }
}

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

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


  
int main() {
  int min_cost = 0;
  int n;
  cin >> n;


  for (int i = 0; i < n; i++) {
    roots[i] = i; // initialize roots
    for (int j = 0; j < n; j++) {
      
      int weight;
      cin >> weight;
      if (j <= i ) continue;
      if (weight != -1) {
	link new_link = {i, j, weight};
	links.push(new_link);
	//cout << "link added has i j weight: " << i << " " << j << " " << weight << endl;
      }
    }
  }

  while (!links.empty()) {
    link cur_link = links.top();
    //cout << "cur link weight: " << cur_link.weight << " x: " << cur_link.x << " y: " << cur_link.y << endl;
    if (!same(cur_link.x, cur_link.y)) {
      min_cost += cur_link.weight;
      unite(cur_link.x, cur_link.y);
    }
    links.pop();
  }

  cout << min_cost << endl;
      
    
}