Go: package main

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

const (
	WHITE int = iota
	GRAY
	BLACK
)

type graph struct {
	G [][]int
	C []int
	D []int
	P []int
}

var INFTY = 1 << 32

func constructGraph() *graph {
	sc := bufio.NewScanner(os.Stdin)
	sc.Scan()
	n, _ := strconv.Atoi(sc.Text())

	g := make([][]int, n)
	c := make([]int, n)
	d := make([]int, n)
	p := make([]int, n)

	for i := 0; i < n; i++ {
		sc.Scan()
		fields := strings.Fields(sc.Text())
		g[i] = toIntSlice(fields)
	}

	return &graph{g, c, d, p}
}

func (g *graph) initMst() {
	for i := 0; i < len(g.G); i++ {
		g.C[i] = WHITE
		g.D[i] = INFTY
	}
	g.D[0] = 0
	g.P[0] = -1
}

func (g *graph) primMst() int {
	g.initMst()

	var u int
	for {
		mincost := INFTY
		for i := 0; i < len(g.G); i++ {
			if g.C[i] != BLACK && g.D[i] < mincost {
				mincost = g.D[i]
				u = i
			}
		}

		if mincost == INFTY {
			break
		}

		g.C[u] = BLACK

		for i := 0; i < len(g.G); i++ {
			if g.C[i] != BLACK && g.existEdge(u, i) {
				if g.G[u][i] < g.D[i] {
					g.D[i] = g.G[u][i]
					g.P[i] = u
				}
			}
		}
	}

	totalCost := 0
	for _, cost := range g.D {
		totalCost += cost
	}
	return totalCost
}

func (g *graph) existEdge(u, v int) bool {
	return g.G[u][v] != INFTY && g.G[v][u] != INFTY
}

func toIntSlice(s []string) []int {
	a := make([]int, len(s))
	for i, v := range s {
		num, _ := strconv.Atoi(v)
		if num == -1 {
			num = INFTY
		}
		a[i] = num
	}
	return a
}

func main() {
	g := constructGraph()
	fmt.Println(g.primMst())
}

C++: #include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

int n; // number of vertices 
int P[20200], A[20200], B[20200], C[20200];
pair<int, int> D[20200]; // Weights and coordinates 

int root(int a) { // find the root of a
  if (P[a] == a) return a; // a is the root
  return  (P[a] = root(P[a])); // find root of a, make it a's parent
}

bool is_same_set(int a, int b) { // a and b are in the same group?
  return root(a) == root(b);
}

void unite(int a, int b) { // put a and b in same group
  P[root(a)] = root(b);
}


int main() {
  cin >> n; // the number of vertices
  for (int i=0; i<n; ++i) {
    P[i] = i; // Initialize vertices to disjoint sets
    for (int j=0; j<n; ++j) {
      A[(n * i) + j] = i;
      B[(n * i) + j] = j;
      cin >> C[(n * i) + j];
    }
  }
  for (int i=0; i < n*n; i++) {
    D[i].first = C[i];
    D[i].second = i;
  }
  sort(D, D+n*n);
  int sum = 0;

  for (int i=0; i<n*n; ++i) {
    if (D[i].first >= 0) { // Assuming the weight is not -1
      if (!is_same_set(A[D[i].second], B[D[i].second])) {
	unite(A[D[i].second], B[D[i].second]);
	sum += D[i].first;
      }
    }
  }
  cout << sum << endl;
}

C++: #include <iostream>

using namespace std;

#define N_MAX 100
#define W_MAX 2000

int Adj[N_MAX][N_MAX] = {};

int color[N_MAX] = {};

int sum_mst_path = 0;
int n;

int prim(void);

int main(){
    int i, j;

    cin >> n;
    for (i = 0; i < n; i++) {
        for (j = 0; j < n; j++) {
            cin >> Adj[i][j];
        }
    }

    color[0] = 1;
    while ( prim() );

    cout << sum_mst_path << endl;

    return 0;
}

//[?????????
//0 : MST????§???????????????????
//1 : ???????????????????????????
int prim(void) {
    int i, j, min, next_node;

    min = W_MAX + 1;
    next_node = N_MAX + 1;

    //?¢????node???????????????????¢????node??????????????§???????°??????¢???path?????¢?´¢
    for (i = 0; i < n; i++) {
        if (color[i] != 1)
            continue;

        for (j = 0; j < n; j++) {
            if (color[j] != 0)
                continue;

            if (Adj[i][j] != -1 && Adj[i][j] < min) {
                min = Adj[i][j];
                next_node = j;
            }
        }
    }

    if (next_node == N_MAX + 1) //??????????¢??????????
        return 0;

    //next_node????¢????
    color[next_node] = 1;
    sum_mst_path += min;

    return 1;
}

C++: #include <algorithm>
#include <iostream>
#include <vector>
#include <math.h>
#include <set>
#include <map>
#include <string>
#include <stack>
#include <queue>
#include <iomanip>
#include <numeric>
#include <tuple>
#define _USE_MATH_DEFINES
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef tuple<int, int, int> tiii;
double pi = 3.141592653589793;
ll mod = 1000000007;
int intmax = 2147483647;
int intmin = -2147483648;
ll llmax = 9223372036854775807;
ll llmin = -9223372036854775807;

struct unionFind {
    vector<int> data;
    unionFind(int size) : data(size, -1) {}
    
    bool unite(int x, int y) {
        x = root(x);
        y = root(y);
        if (x != y) {
            if(data[y] < data[x]) {
                swap(x, y);
            }
            data[x] += data[y];
            data[y] = x;
        }
        return x != y;
    }

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

    int root(int x) {
        return (data[x] < 0) ? x : data[x] = root(data[x]);
    }

    int rank(int x) {
        return -data[root(x)];
    }
};

//cost,from,toの順
vector<tiii> T;
vector<tiii> Krusans;

ll Kruskal(int n) {
    sort(T.begin(), T.end());
    ll ans = 0;
    ll num = 0;
    unionFind u(n);
    int size = T.size();
    for (int i = 0; i < size; i++) {
        int cost = get<0>(T[i]);
        int from = get<1>(T[i]);
        int to = get<2>(T[i]);
        if (u.root(from) != u.root(to)) {
            u.unite(from, to);
            ans += cost;
            num++;
            tiii t = make_tuple(cost, from, to);
            Krusans.push_back(t);
        }
        if (num == n - 1) {
            break;
        }
    }
    return ans;
}

int mat[200][200];

int main() {
    int N;
    cin >> N;
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            cin >> mat[i][j];
        }
    }
    for (int i = 0; i < N; i++) {
        for (int j = i + 1; j < N; j++) {
            if (mat[i][j] != -1) {
                tiii e = make_tuple(mat[i][j], i, j);
                T.push_back(e);
            }
        }
    }
    ll ans = Kruskal(N);
    cout << ans << endl;
}

C++: //include
//------------------------------------------
#include <iostream>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <limits>
using namespace std;

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define FORR(i,a,b) for(int i=(a);i>(b);i--)
#define REP(i,n)  FOR(i,0,n)

//conversion
//------------------------------------------
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();}

//debug
//-------------------------------------------
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;
#define INF std::numeric_limits<int>::max()

//typedef
//-------------------------------------------
typedef long long ll;

int n;
int g[101][101];

int prim(int used, int sum) {
    if (used == 0) return sum;

    int tmp = INF;
    int use = -1;
    REP(i, n) {
        if ((used>>i)&1) {
            continue;
        }
        REP(j, n) {
            if (((used>>j)&1) && g[i][j] < tmp) {
                tmp = g[i][j];
                use = j;
            }
        }
    }
    return prim(used&~(1<<use), sum + tmp);
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    cin >> n;
    REP(i, n) REP(j, n) {
        cin >> g[i][j];
        if (g[i][j] < 0) g[i][j] = INF;
    }
    int used = (1<<n) - 1;
    cout << prim(used&~1, 0) << endl;

    return 0;
}