Go: package main

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

func getScanner(fp *os.File) *bufio.Scanner {
	scanner := bufio.NewScanner(fp)
	scanner.Split(bufio.ScanWords)
	scanner.Buffer(make([]byte, 500001), 500000)
	return scanner
}

func getNextString(scanner *bufio.Scanner) string {
	scanner.Scan()
	return scanner.Text()
}

func getNextInt(scanner *bufio.Scanner) int {
	i, _ := strconv.Atoi(getNextString(scanner))
	return i
}

func main() {
	fp := os.Stdin
	if len(os.Args) > 1 {
		fp, _ = os.Open(os.Args[1])
	}

	scanner := getScanner(fp)
	writer := bufio.NewWriter(os.Stdout)

	n := getNextInt(scanner)

	g := make([][]int, n)
	ans := make([][]int, n)

	inf := 1 << 30
	for i := 0; i < n; i++ {
		g[i] = make([]int, n)
		ans[i] = make([]int, n)
		for ii := 0; ii < n; ii++ {
			g[i][ii] = getNextInt(scanner)
			if g[i][ii] == -1 {
				g[i][ii] = inf
			}
			ans[i][ii] = -1
		}
	}

	im := make(IntMap, 1)

	im.increment(1)

	for len(im) < n {
		min := inf
		u := -1
		v := -1
		for i := range im {
			for ii := 0; ii < n; ii++ {
				if im.get(ii) != 0 {
					continue
				}
				if min > g[i][ii] {
					min = g[i][ii]
					u = i
					v = ii
				}
			}
		}
		im.increment(v)
		ans[u][v] = min
		ans[v][u] = min
	}

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

	fmt.Fprintln(writer, sum)
	writer.Flush()
}

// IntMap ...
type IntMap map[int]int

func (m IntMap) get(key int) int {
	if _, ok := m[key]; ok {
		return m[key]
	}
	return 0
}
func (m IntMap) increment(key int) {
	if _, ok := m[key]; ok == false {
		m[key] = 0
	}
	m[key]++
}

C++: #include <iostream> // cout, endl, cin
#include <string> // string, to_string, stoi
#include <vector> // vector
#include <algorithm> // min, max, swap, sort, reverse, lower_bound, upper_bound
#include <utility> // pair, make_pair
#include <tuple> // tuple, make_tuple
#include <map> // map
#include <queue> // queue, priority_queue
#include <set> // set
#include <stack> // stack
#include <deque> // deque
#define ll long long
#define ALL(x) (x).begin(),(x).end()
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define vi vector<int>
#define vvi vector<vector<int>>
using namespace std;

class UnionFind {
private:
    vector<vector<int>> Graph; // {ID,Number,ParentID,Height}

    void flatten(int x, int rootid) {
        int parentid = Graph[x][0];
        while (Graph[parentid][0] != Graph[parentid][2]) {
            Graph[parentid][2] = rootid;
            int next = Graph[parentid][2];
            parentid = next;
        }
    }
public:
    void push(int x) {
        int i = Graph.size();
        Graph.push_back({ i,x,i,1 });
    }
    void unite(int x, int y) {
        int hx = Graph[root(x)][3];
        int hy = Graph[root(y)][3];
        if (hx < hy) {
            Graph[root(x)][2] = root(y);
            flatten(x, root(y));
        }
        else if(hx == hy){
            Graph[root(y)][2] = root(x);
            Graph[root(x)][3]++;
            flatten(y, root(x));
        }
        else {
            Graph[root(y)][2] = root(x);
            flatten(y, root(x));
        }
    }
    bool same(int x, int y) {
        return root(x) == root(y);
    }

   
    int root(int x) {
        int parentid = Graph[x][0];
        while (Graph[parentid][0] != Graph[parentid][2]) {
            int next = Graph[parentid][2];
            parentid = next;
        }
       
        return parentid;
    }
};

int main() {
    int n; cin >> n;
    vector<vector<int>> edges; // {weight, from_id, to_id} both 0-indexed
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            int w; cin >> w;
            if (w == -1) {
                continue;
            }
            else {
                vector<int> temp_edge = { w,i,j };
                edges.push_back(temp_edge);
            }
        }
    }

    sort(ALL(edges));
    int n_edges = 0;
    int TotalCost = 0;

    UnionFind uf;
    for (int i = 0; i < n; i++) uf.push(i);

    for (int i = 0; n_edges < n - 1; i++) {
        vector<int> edge = edges[i];
       
        if (uf.root(edge[1]) != uf.root(edge[2])) {
            uf.unite(edge[1], edge[2]);
            n_edges++;
            TotalCost += edge[0];
        }
    }


    cout << TotalCost << endl;
}

C++: #include<iostream>
using namespace std;
static const int MAX = 100;
static const int INFTY = (1<<21);
static const int WHITE = 0;
static const int GRAY = 1;
static const int BLACK = 2;

int n, N[MAX][MAX];

int prim()
{
  int u,minv;
  int d[MAX],p[MAX],color[MAX];
  int i=0;
  int v=0;
  int j=0;

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

  d[0] = 0;

  while(1)
    {
      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 && N[u][v]!=INFTY)
	    {
	      if(d[v]>N[u][v])
		{
		  d[v] = N[u][v];
		  p[v] = u;
		  color[v] = GRAY;
		}
	    }
	}
    }
  int sum=0;
  for(i=0;i<n;i++)
    {
      if(p[i]!=-1)
	{
	  sum += N[i][p[i]];
	}
    }

      return sum;
    }

  int main()
  {
    int i=0;
    int j=0;
    cin>>n;

    for(i=0;i<n;i++)
      {
	for( j=0 ; j<n ; j++ )
	  {
	    int e;
	    cin>>e;
	    N[i][j] = (e == -1)? INFTY : e;
	  }
      }

    cout<<prim()<<endl;

    return 0;
  }

C++: #include <bits/stdc++.h>
using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP0(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)
#define REP1(i,n) for(int i=1, i##_len=(n); i<=i##_len; ++i)
#define RREP0(i,n) for(int i=n-1; i>=0; --i)
#define RREP1(i,n) for(int i=n; i>=1; --i)

#define SORT(c) sort((c).begin(),(c).end())
#define whole(f,x,...) ([&](decltype((x)) whole) { return (f)(begin(whole), end(whole), ## __VA_ARGS__); })(x)
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define CLR(a) memset((a), 0 ,sizeof(a))

inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}
int gcd(int a,int b){return b?gcd(b,a%b):a;}
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};

typedef long long LL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;

const int INTINF = 2147483647;
const LL LLINF = 9223372036854775807;
const double EPS = 1e-10;
const double PI  = acos(-1.0);
const int N = 100;
int color[N]; // i's visit status
int M[N][N];
int d[N];  // minimum weight
int p[N]; // i's parent vertex in MST
int n;

#define WHITE 0
#define GRAY 1
#define BLACK 2

struct init{
    init(){
        cin.tie(0); ios::sync_with_stdio(0); cout<<fixed<<setprecision(20);
    };
} init;


void prim(){
  int u, minv;

  REP0(i,n){
    color[i] = WHITE;
    d[i] = INTINF;
    p[i] = -1;
  }

  d[0] = 0;

  while (1) {
    minv = INTINF;
    u = -1;
    REP0(i,n){
      if( minv > d[i] && color[i] != BLACK ){
        u = i;
        minv = d[i];  // MSTの隣接辺のうち最小重みを選ぶ
      }
    }
    if ( u==-1 ) break; // 最小重みがなく終了
    color[u] = BLACK; // uをMSTに追加
    REP0(v,n){ // 新たなMSTで, 隣接する頂点の重みと親を更新
      if ( color[v] != BLACK && M[u][v] != INTINF  && d[v] > M[u][v] ) {
        d[v] = M[u][v];
        p[v] = u;
        color[v] = GRAY;
      }
    }
  }

 int sum = 0;
 REP0(i,n){ // MST内で親を持つ頂点について
   if( p[i] != -1 ) sum += M[i][p[i]];
 }
 std::cout << sum << '\n';
}


void input(){
  std::cin >> n;
  int w;
  REP0(i,n) REP0(j,n) {
    std::cin >> w;
    M[i][j] = (w==-1) ? INTINF : w;
  }
}


int main(int argc, char const *argv[]) {
  input();
  prim();
  return 0;
}