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

#define rep(i, n) REP(i, 0, n)
#define ALL(v) v.begin(), v.end()
#define MSG(a) cout << #a << " " << a << endl;
#define REP(i, x, n) for (int i = x; i < n; i++)
#define OP(m) cout << m << endl
typedef long long ll;
typedef unsigned long long ull;

int main()
{
    int n, sum = 0;
    cin >> n;
    int m[n][n];
    rep(i, n) rep(j, n) cin >> m[i][j];

    rep(i, n) rep(j, n) if (m[i][j] == -1) m[i][j] = 1e7;

    int d[n];
    rep(i, n) d[i] = 1e7;
    bool used[n] = {};
    d[0] = 0;
    while (1)
    {

        int minv = 1e7;
        int u = -1;
        rep(i, n)
        {
            if (!used[i] && d[i] < minv)
            {
                minv = d[i];
                u = i;
            }
        }
        if (u == -1)
            break;

        sum += minv;
        used[u] = true;

        rep(i, n)
        {

            if (!used[i] && m[u][i] != 1e7 && d[i] > m[u][i])
                d[i] = m[u][i];
        }
    }

    OP(sum);

    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define _GLIBCXX_DEBUG
typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<pair<int, int>, int> PP;
const ll INF = 1LL << 60;
const ll MOD = 1000000007;
const int MAX = 100000;

int GCD(int a, int b) { return b ? GCD(b, a % b) : a; }
//------------------------------------------------------------------------------------------------------------------------------

vector<int> par(MAX);
vector<int> rk(MAX);
vector<int> cnt(MAX);

void init(int n) {
    for (int i = 0; i < n; i++) {
        par[i] = i;
        rk[i] = 0;
        cnt[i] = 1;
    }
}

int find(int x) {
    if (par[x] == x) {
        return x;
    } else {
        return par[x] = find(par[x]);
    }
}

void unite(int x, int y) {
    x = find(x);
    y = find(y);
    if (x == y)
        return;

    if (rk[x] < rk[y]) {
        par[x] = y;
        cnt[x] += cnt[y];
    } else {
        par[y] = x;
        cnt[y] += cnt[x];
        if (rk[x] == rk[y])
            rk[x]++;
    }
}

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

struct edge {
    int u, v, cost;
};

bool comp(const edge &e1, const edge &e2) { return e1.cost < e2.cost; }
vector<edge> es;
int kruskal(int V, int E) {
    sort(es.begin(), es.end(), comp);
    init(V);
    int res = 0;
    for (int i = 0; i < E; i++) {
        edge e = es[i];
        if (!same(e.u, e.v)) {
            unite(e.u, e.v);
            res += e.cost;
        }
    }
    return res;
}

int main(void) {
    int N;
    cin >> N;
    int E = 0;
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            int c;
            cin >> c;
            if (c >= 0) {
                es.push_back({i, j, c});
                E++;
            }
        }
    }
    cout << kruskal(N, E) << endl;
}

C++: #include<iostream>

using namespace std;

static const int MAX = 100;
static const int NIL = (1 << 21);
static const int WHITE = 0;
static const int GREY = 1;
static const int BLACK = 2;


int n, M[MAX][MAX];;

int prim() {
  int u; minv;
  int d[MAX], p[MAX], color [MAX]

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

  d[0] = 0;

  while (1) {
    minv = INFTY;
    u = -1;
    for (int 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 (int v = 0; i < n; i++) {
      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;
        }
      }
    }
  }
  int sum = 0;
  for (int i = 0; i < n; i++) {
    if (p[i] != -1) {
      sum += M[i][p[i]]
    }
  }

  return sum;
}

int main() {
  cin >> n;

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

  cout << prim() << endl;

  return 0;
}