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 <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;
}