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 <utility>
#include <iostream>
#include <algorithm>
using namespace std;

int root(int a,int P[])
{
    if(P[a] == a)
        return a;
    else    return (P[a] = root(P[a],P));
}
int is_same_set(int a,int b,int P[])
{
    return root(a,P) == root(b,P);
}

int main()
{
    int n,i,j,M=0,minsum=0;
    cin >> n;
    //init(n);
    int a[n][n],cost[n*n],src[n*n],dst[n*n];
    for(i=0;i<n;i++)
        for(j=0;j<n;j++)
            cin >> a[i][j];
    for(int r=0;r<n;r++)
        for(int c=r+1;c<n;c++)
            if(a[r][c] > -1)
            {
                cost[M] = a[r][c];
                src[M] = r;
                dst[M] = c;
                M++;
            }
    pair<int,int> edeges[M];
    int P[n];
    for(i=0;i<n;i++)
        P[i] = i;
    for(i=0;i<M;i++)
    {
        edeges[i].first = cost[i];
        edeges[i].second = i;
    }
    sort(edeges,edeges+M);
    for(i=0;i<M;i++)
    {
        if(is_same_set(src[edeges[i].second], dst[edeges[i].second], P))
            continue;
        else    {
                    P[root(src[edeges[i].second],P)] = root(dst[edeges[i].second],P);
                    minsum += edeges[i].first;
                }
    }
    cout << minsum << endl;
    return 0;
}

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

vector< vector < int > > adjMatrix;

int findMSTCost(int n) {
  vector< bool > visited (n, false);
  visited[0] = true;
  int cost = 0;
  for (int numberVertices = 1; numberVertices < n; numberVertices++) {
    int minEdge = 3000, minVertex = n;
    for (int i = 0; i < n; i++) {
      for (int j = 0; j < n; j++) {
	if ((i != j) &&
	    (visited[i] == true) &&
	    (visited[j] == false) &&
	    (adjMatrix[i][j] != -1) &&
	    (adjMatrix[i][j] < minEdge)) {
	  minEdge = adjMatrix[i][j];
	  minVertex = j;
	}
      }
    }
    cost += minEdge;
    visited[minVertex] = true;
  }
  return cost;
}

int main(int argc, char **argv) {
  ios_base::sync_with_stdio(false);
  int n;
  cin >> n;
  adjMatrix = vector< vector < int > > (n, vector< int > (n, 0));
  for (int i = 0; i < n; i++)
    for (int j = 0; j < n; j++)
      cin >> adjMatrix[i][j];
  cout << findMSTCost(n) << endl;
  return 0;
}

C++: #include <bits/stdc++.h>

using namespace std;

typedef long long int ll;
typedef pair<ll, ll> pll;

#define FOR(i, n, m) for(ll (i)=(m);(i)<(n);++(i))
#define REP(i, n) FOR(i,n,0)
#define OF64 std::setprecision(10)

const ll MOD = 1000000007;
const ll INF = (ll) 1e15;

struct UnionFind {
    UnionFind(int n) {
        rank.assign(n, 0);
        parent.resize(n);
        for (int i = 0; i < n; ++i) {
            parent[i] = i;
        }
    }

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

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

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

        if (rank[x] < rank[y]) {
            parent[x] = y;
        }
        else {
            parent[y] = x;
            if (rank[x] == rank[y])
                rank[x]++;
        }
    }

    vector<int> rank;
    vector<int> parent;
};

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

    ll N;
    cin >> N;
    priority_queue<tuple<pll, ll>, vector<tuple<pll, ll>>, function<bool(tuple<pll, ll>, tuple<pll, ll>)>> q(
            [](tuple<pll, ll> a, tuple<pll, ll> b) {
                return get<1>(a) > get<1>(b);
            });
    REP(i, N) {
        REP(j, N) {
            ll a;
            cin >> a;
            if (a < 0)
                continue;
            q.push(make_tuple(pll(i, j), a));
        }
    }
    UnionFind uf(N);
    ll ans = 0;
    while (!q.empty()) {
        auto t = q.top();
        q.pop();
        pll p = get<0>(t);
        if (uf.same(p.first, p.second))
            continue;
        uf.unit(p.first, p.second);
        ans += get<1>(t);
    }
    cout << ans << endl;
    return 0;
}

C++: #include <stdio.h>
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, M[MAX][MAX];

int prim() {
    int d[n], p[n];
    int color[n];
    
    for(int i=0; i<n; i++) {
        color[i] = WHITE;
        d[i] = INFTY;
        p[i] = -1;
    }
    
    d[0] = 0;
    
    while(1) {
        int u;
        int mincost = INFTY;
        for (int i=0; i<n; i++) {
            if (color[i] != BLACK && d[i] < mincost) {
                mincost = d[i];
                u = i;
            }
        }
        
        if (mincost == INFTY) break;
        
        color[u] = BLACK;
        
        for (int v=0; v<=n; v++) {
            if (color[v] != BLACK && M[u][v] != INFTY) {
                if(M[u][v] < d[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[p[i]][i];
    }
    return sum;
}

int main() {
    scanf("%d", &n);
    for (int i=0; i<n; i++) {
        for (int j=0; j<n; j++) {
            int tmp;
            scanf("%d", &tmp);
            M[i][j] = (tmp == -1) ? INFTY : tmp;
        }
    }
    
    int sum = prim();
    printf("%d\n", sum);
    
    return 0;
}