Go: package main

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

const (
	MAX   = 100
	INFTY = 1 << 21
	NIL   = -1
)

const (
	WHITE = iota
	GRAY
	BLACK
)

type Graph [MAX][MAX]int

func (g *Graph) prim(n int) int {
	var (
		u, minv     = 0, 0
		d, p, color [MAX]int
	)

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

	for {
		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 && g[u][v] != INFTY {
				if d[v] > g[u][v] {
					d[v] = g[u][v]
					p[v] = u
					color[v] = GRAY
				}
			}
		}
	}

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

	return sum
}

func answer(reader io.Reader) string {
	sc := bufio.NewScanner(reader)
	sc.Split(bufio.ScanWords)

	sc.Scan()
	n, _ := strconv.Atoi(sc.Text())

	var g Graph
	for i := 0; i < n; i++ {
		for j := 0; j < n; j++ {
			sc.Scan()
			e, _ := strconv.Atoi(sc.Text())
			if e == -1 {
				g[i][j] = INFTY
				continue
			}
			g[i][j] = e
		}
	}

	return fmt.Sprint(g.prim(n))
}

func main() {
	fmt.Println(answer(os.Stdin))
}

C++: #include <cstdio>
#include <vector>
#include <algorithm>
#include <queue>

using namespace std;

template <class T>
using lp_queue=priority_queue<T, vector<T>, greater<T>>;

using Weight=int;
using DArray=vector<Weight>;
using DMatrix=vector<DArray>;

static const Weight INF=1<<29;

struct Edge {
    size_t src, dst;
    Weight weight;
    Edge() {}
    Edge(size_t src, size_t dst, Weight weight=1):
        src(src), dst(dst), weight(weight)
    {}
};

bool operator<(const Edge &e, const Edge &f) {
    if (e.weight != f.weight) {
        return e.weight > f.weight;
    } else {
        return e.src!=f.src? e.src<f.src : e.dst<f.dst;
    }
}

using Edges=vector<Edge>;
using Vertex=vector<Edge>;
using Graph=vector<Vertex>;

void join(Graph &g, size_t s, size_t d, Weight w=1) {
    // non-directed
    g[s].push_back(Edge(s, d, w));
    g[d].push_back(Edge(d, s, w));
}

void connect(Graph &g, size_t s, size_t d, Weight w=1) {
    // directed
    g[s].push_back(Edge(s, d, w));
}

Weight minimum_spanning_tree(const Graph &g, size_t s) {
    /* Prim */
    size_t V=g.size();
    vector<Weight> d(V, INF); d[s]=0;
    Weight res=0;

    lp_queue<pair<Weight, size_t>> q; q.push(make_pair(0, s));
    while (!q.empty()) {
        pair<Weight, size_t> p=q.top(); q.pop();
        size_t v=p.second;
        if (d[v] < p.first) continue;

        res += d[v];
        for (const Edge &e: g[v])
            if (d[e.dst] > e.weight) {
                d[e.dst] = e.weight;
                q.push(make_pair(d[e.dst], e.dst));
            }
    }
    return res;
}

int main() {
    size_t n;
    scanf("%zu", &n);

    Graph g(n);
    for (size_t i=0; i<n; ++i)
        for (size_t j=0; j<n; ++j) {
            Weight a;
            scanf("%d", &a);

            if (a > -1)
                connect(g, i, j, a);
        }

    printf("%d\n", minimum_spanning_tree(g, 0));
    return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

int G[110][110];
int cost[1000],src[1000],dst[1000];

int P[10010];
int root(int a);
int is_same_set(int a,int b);
void unite(int a,int b);

int main() {
	int N,a;
	cin >> N;
	for(int i=0; i<N;i++) {
		for(int j=0;j<N;j++) {
			cin >> a;
			G[i][j] = a;
		}
	}
	int hcon = 1;
	int M = 0;
	pair<int,int> edges[10000];

	for(int r=0;r<N;r++) {
		for (int c=0;c<hcon;c++) {
			if(G[r][c] > -1) {
				cost[M] = G[r][c];
				src[M] = r;
				dst[M] = c;
				edges[M] = make_pair(cost[M],M);
				M+=1;
			}
		}
		hcon++;
	}
	sort(edges,edges+M);



	for(int i=0;i<N;i++) P[i] = i;

	int answer = 0;
	for(int i=0;i<M;i++) {
		if(!is_same_set(src[edges[i].second],dst[edges[i].second])) {
			unite(src[edges[i].second],dst[edges[i].second]);
			answer += edges[i].first;
			//cout << answer << endl;
		}
	}
	cout << answer << endl;;
}

int root(int a) { 
	if (P[a] == a) return a; 
	return (P[a] = root(P[a])); 
}
int is_same_set(int a, int b) { 
	if(root(a) == root(b)) {
		return 1;
	} else {
		return 0;
	}
}
void unite(int a, int b) { 
	P[root(a)] = root(b);
}

C++: #include <iostream>
#include <climits>
#include <queue>

using namespace std;
static const int MAX=100;
static const int WHITE=1;
static const int GRAY=2;
static const int BLACK=3;

int n,M[MAX+10][MAX+10];

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

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

    d[0]=0;

    while(1)
    {
        minv=INT_MAX;
        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;v<n;v++)
      {
        if(color[v]!=BLACK && M[u][v]!=INT_MAX)
        {
            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)? INT_MAX:e;
    }

    cout<<prim()<<endl;
    return 0;
}

C++: #include <stdio.h>
#include <iostream>
#include <utility>
#include <algorithm>
#include <math.h>
#include <vector>
#define N 10000
using namespace std;

class edge
{
  public:
    int i, j, weigt;
    bool operator()(edge e1, edge e2)
    {
        return e1.weigt < e2.weigt;
    }
};
edge e[N];
vector<int> adj[N];
int n;
int c = 0;
int f[N] = {};
int g[N] = {};

void setGraphNum(int v, int i)
{
    g[v] = i;
    f[v]++;
    vector<int>::iterator itr = adj[v].begin();
    while (itr != adj[v].end())
    {
        if (f[*itr] != c)
            setGraphNum(*itr, i);
        itr++;
    }
}
void CalcGraphNum()
{
    int k = 1;
    for (int i = 0; i < n; i++)
    {
        if (f[i] != c)
            setGraphNum(i, k++);
    }
}
int main()
{
    cin >> n;
    int k = 0, tmp = 0, sum = 0;
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            cin >> tmp;
            if (j < i + 1 || tmp < 0)
                continue;
            e[k].i = i;
            e[k].j = j;
            e[k].weigt = tmp;
            k++;
        }
    }
    sort(e, e + k, edge());
    for (int i = 0; i < n; i++)
        g[i] = i;
    for (int i = 0; i < k; i++)
    {
        if (g[e[i].i] == g[e[i].j])
            continue;
        adj[e[i].i].push_back(e[i].j);
        adj[e[i].j].push_back(e[i].i);
        sum += e[i].weigt;
        //cout << e[i].i << " " << e[i].j << " " << e[i].weigt << "\n";
        if (++c == n - 1)
            break;
        CalcGraphNum();
    }

    cout << sum << "\n";
    return 0;
}