Go: package main

import (
	"bufio"
	"fmt"
	"log"
	"math"
	"os"
	"strconv"
	"strings"
)

type color int

const (
	white color = iota // 未訪問
	mst                // MSTとして確定
)

// 頂点
type node struct {
	matrix   []int // 隣接行列
	minToMst int   // MSTへの重み最小値
	parent   int   // 最小値の相手
	color    color
}

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	n := eGetInt(scanner)

	nodes := make([]node, n)
	for i := 0; i < n; i++ {
		nodes[i].matrix = eAtoi((strings.Fields(eGetline(scanner))))
		nodes[i].parent = -1
		nodes[i].minToMst = math.MaxInt16
		nodes[i].color = white
	}

	// 初回はnode[0]がMSTに入るように,minToMstを設定
	nodes[0].minToMst = 0
	for { // 全部チェックするまで

		// どの頂点を新たにMSTに取り込むか
		min := math.MaxInt16
		u := -1
		for i := 0; i < n; i++ {
			if nodes[i].color == mst {
				continue
			}
			if nodes[i].minToMst < min {
				u = i
				min = nodes[i].minToMst
			}
		}

		if u == -1 {
			break
		}
		// MSTに追加する頂点決定
		nodes[u].color = mst

		// MSTに属していない頂点それぞれについて,MSTとの最小の重みを更新
		for i := 0; i < n; i++ {
			if nodes[i].color == mst {
				continue
			}
			if nodes[i].matrix[u] == -1 {
				// 接してない
				continue
			}
			if nodes[i].matrix[u] < nodes[i].minToMst {
				// node[u]との重みが最小
				nodes[i].minToMst = nodes[i].matrix[u]
				nodes[i].parent = u
			}
		}
	}

	sum := 0
	for _, v := range nodes {
		// node[0]はparentが入っていない
		if v.parent != -1 {
			sum += v.matrix[v.parent]
		}
	}
	fmt.Println(sum)
}

func eGetInt(scanner *bufio.Scanner) int {
	t := eAtoi(strings.Fields(eGetline(scanner)))
	return t[0]
}

func eGetline(p *bufio.Scanner) string {
	if !p.Scan() {
		if err := p.Err(); err != nil {
			log.Fatal(err.Error())
		}
		return ""
	}
	return p.Text()
}

func eAtoi(s []string) []int {
	ints := make([]int, len(s))
	var err error
	for i, e := range s {
		ints[i], err = strconv.Atoi(e)
		if err != nil {
			log.Fatal(err.Error())
		}
	}
	return ints
}

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

class Graph
{
 private:
  std::vector<std::vector<long> > weight_;
 public:
  Graph(const unsigned long node_num){};


  void insert_weight(const std::vector<long> weight){
    weight_.push_back(weight);
  };


  void display()
  {
    for(unsigned long i = 0, imax = weight_.size(); i < imax; ++i)
    {
      for(unsigned long j = 0, jmax = weight_[i].size(); j < jmax; ++j)
      {
        std::cout << " " << weight_[i][j];
      }
      std::cout << std::endl;
    }
  };


  unsigned long MinimumSpanningTree()
  {
    if(weight_.size() <= 1) return 0;

    struct Edge
    {
      long value_;
      long start_node_;
      long end_node_;
      Edge(long value = LONG_MAX, long sn = -1, long en = -1) : value_(value), start_node_(sn), end_node_(en){};
      void change(long value, long sn, long en)
      {
        value_ = value; start_node_ = sn, end_node_ = en;
      };
      void display()
      {
        std::cout << value_ << " : " << start_node_ << " : " << end_node_ << std::endl;
      };
    };

    unsigned long minimum_spanning_value = 0;

    std::set<long> connect_node; connect_node.insert(0);
    std::set<long> noconnect_node;

    for(unsigned long node_i = 1, imax = weight_.size(); node_i < imax; ++node_i)
      noconnect_node.insert(node_i);

    while(!noconnect_node.empty())
    {
      Edge min_edge;
      for(std::set<long>::iterator it = connect_node.begin(); it != connect_node.end(); ++it)
      {
        for(std::set<long>::iterator noit = noconnect_node.begin(); noit != noconnect_node.end(); ++noit)
        {
          if(weight_[*it][*noit] == -1) continue;
          if(weight_[*it][*noit] < min_edge.value_)
          {
            min_edge.value_ = weight_[*it][*noit];
            min_edge.change(weight_[*it][*noit], *it, *noit);
          }
        }
      }
      connect_node.insert(min_edge.end_node_);
      noconnect_node.erase(min_edge.end_node_);
      minimum_spanning_value += min_edge.value_;
    }

    return minimum_spanning_value;
  };//   unsigned long MinimumSpanningTree()
 protected:
};//    class Graph

int main()
{

  unsigned long node_num = 0;
  std::cin >> node_num; std::cin.ignore();

  Graph graph(node_num);

  for(unsigned long node_i = 0; node_i < node_num; ++node_i)
  {
    std::vector<long> weight(node_num, -1);
    for(unsigned long weight_i = 0, weight_num = node_num; weight_i < weight_num; ++weight_i)
    {
      std::cin >> weight[weight_i]; std::cin.ignore();
    }
    graph.insert_weight(weight);
  }

  std::cout << graph.MinimumSpanningTree() << std::endl;

  return 0;
};//    main()

C++: #include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <functional>
#include <tuple>
#include <sstream>

struct Node
{
  int data;
  Node* parent;
  int rank;
  Node() : data(-1), parent(this), rank(0){}
  Node& operator=(int data){ this->data = data; return *this; }
  Node* Find(){
    if (this == this->parent){
      return this;
    } else {
      auto root = this->parent->Find();
      this->parent = root;
      return root;
    }
  }
  static void Union(Node& lhs, Node& rhs){
    Node* lhs_root = lhs.Find();
    Node* rhs_root = rhs.Find();
    if (lhs_root->rank > rhs_root->rank){
      rhs_root->parent = lhs_root;
    } else if (rhs_root->rank > lhs_root->rank){
      lhs_root->parent = rhs_root;
    } else if (rhs_root != lhs_root){
      lhs_root->parent = rhs_root;
      rhs_root->rank += 1;
    }
    return;
  }
};

bool is_not_space(int c){ return c != ' '; }
std::string& ltrim(std::string& str){
  str.erase(str.begin(), std::find_if(str.begin(), str.end(), is_not_space));
  return str;
}
std::string& rtrim(std::string& str){
  str.erase(std::find_if(str.rbegin(), str.rend(), is_not_space).base(), str.end());
  return str;
}
std::string trim(std::string& str){ return ltrim(rtrim(str)); }

struct Edge{
  int i,j,w;
  bool exist;
  Edge(int i,int j,int w): i(i), j(j), w(w), exist(true){ if(w==-1){ this->exist = false; } }
  int Value(){ return this->exist ? this->w : 2005; }
};
int main(void){
  std::string line;
  std::getline(std::cin, line);
  int n = std::stoi(line);
  std::vector<Node> V(n);
  std::vector<Edge> E;
  for(int i = 0; i < n; ++i){
    std::getline(std::cin, line);
    std::stringstream ss(trim(line));
    for(int j = 0; j < n; ++j){
      int w;
      ss >> w;
      E.emplace_back(i,j,w);
    }
  }
  int sum_w = 0;
  for(int remain = n-1; remain > 0;){
    auto min_elem = std::min_element(E.begin(), E.end(), [](auto& lhs, auto& rhs){ return lhs.Value() < rhs.Value(); });
    min_elem->exist = false;
    E[min_elem->j * n + min_elem->i].exist = false;
    if(V[min_elem->i].Find() != V[min_elem->j].Find()){
      Node::Union(V[min_elem->i], V[min_elem->j]);
      sum_w += min_elem->w;
      remain -= 1;
    }
  }
  std::cout << sum_w << std::endl;
  return 0;
}