Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

type ElementKey int
type Node struct {
	key    ElementKey
	parent *Node
	rank   int
}

func newNode(key ElementKey) *Node {
	node := Node{}
	node.key = key
	node.parent = &node
	node.rank = 0

	return &node
}

func union(x, y *Node) {
	xSet := findSet(x)
	ySet := findSet(y)
	link(xSet, ySet)
}

func link(x, y *Node) {
	if x.rank > y.rank {
		y.parent = x
	} else {
		x.parent = y

		if x.rank == y.rank {
			y.rank++
		}
	}
}

func findSet(x *Node) *Node {
	if x != x.parent {
		x.parent = findSet(x.parent)
	}
	return x.parent
}

func newDisjointSets(nSets int) *DisjointSets {
	sets := DisjointSets{}
	sets.keyToNode = map[ElementKey]*Node{}
	for i := 0; i < nSets; i++ {
		key := ElementKey(i)
		sets.keyToNode[key] = newNode(key)
	}
	return &sets
}

type DisjointSets struct {
	keyToNode map[ElementKey]*Node
}

func (sets *DisjointSets) union(x, y ElementKey) {
	xNode := sets.keyToNode[x]
	yNode := sets.keyToNode[y]
	union(xNode, yNode)
}

func (sets DisjointSets) findSet(x ElementKey) *Node {
	xNode := sets.keyToNode[x]
	return findSet(xNode)
}

type Edge struct {
	i, j   int
	weight int
}

func main() {
	scanner.Split(bufio.ScanWords)

	n := nextInt()
	sets := newDisjointSets(n)
	edges := []Edge{}
	a := make([][]int, n)
	for i := 0; i < n; i++ {
		a[i] = make([]int, n)
		for j := 0; j < n; j++ {
			a[i][j] = nextInt()
			if a[i][j] >= 0 {
				e := Edge{
					i:      i,
					j:      j,
					weight: a[i][j],
				}
				edges = append(edges, e)
			}
		}
	}
	sort.Slice(edges, func(i, j int) bool { return edges[i].weight < edges[j].weight })
	totalWeight := 0
	for _, edge := range edges {
		i := ElementKey(edge.i)
		j := ElementKey(edge.j)
		if sets.findSet(i) != sets.findSet(j) {
			totalWeight += edge.weight
			sets.union(i, j)
		}
	}
	fmt.Println(totalWeight)
}

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

using namespace std;

const int maxn=1e2+5;
const int inf=(1<<21);
const int white=0;
const int gray=1;
const int black=2;

int n,m[maxn][maxn];

int prim()
{
    int d[maxn],p[maxn],book[maxn];
    for(int i=0;i<n;i++){
        d[i]=inf;
        p[i]=-1;
        book[i]=0;
    }
    d[0]=0;
    int u,minv,sum=0;
    while(1){
        minv=inf;
        u=-1;
        for(int i=0;i<n;i++){
            if(minv>d[i]&&book[i]==0){
                u=i;
                minv=d[i];
            }
        }
        if(u==-1){
            break;
        }else{
            sum+=d[u];
        }
        book[u]=1;
        for(int v=0;v<n;v++){
            if(book[v]==0&&d[v]>m[u][v]){
                d[v]=m[u][v];
                p[v]=u;
            }
        }
    }
//    int sum=0;
//    for(int i=0;i<n;i++){
//        if(p[i]!=-1){
//            sum+=m[i][p[i]];
//        }
//    }
    return sum;
}

int main()
{
    scanf("%d",&n);
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            int e;
            cin>>e;
            m[i][j]=(e==-1)?inf:e;
        }
    }
    cout<<prim()<<endl;
    return 0;
}
/*
5
-1 2 3 1 -1
2 -1 -1 4 -1
3 -1 -1 1 1
1 4 1 -1 3
-1 -1 1 3 -1
*/

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

using namespace std;
typedef pair<int, int> P;   //cost, to
static const int INF = 1e9;
static const int SIZE = 101;
int par[SIZE];
int rk[SIZE];
int d[SIZE];
int n;

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

struct edge2
{
	int to, cost;
};

bool comp(const edge& e1, const edge& e2)
{
	return(e1.cost < e2.cost);
}

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

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

void unite(int a, int b)
{
	int x = find(a);
	int y = find(b);
	if (x == y)
	{
		return;
	}
	if (rk[x] < rk[y])
	{
		par[x] = y;
	}
	else
	{
		par[y] = x;
		if (rk[x] == rk[y])
		{
			++rk[x];
		}
	}
}

bool same(int a, int b)
{
	return(find(a) == find(b));
}

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

int prim(vector<edge2>* G)
{
	vector<bool> isUsed(n);
	for (int i = 0; i < n; ++i)
	{
		isUsed[i] = false;
	}
	priority_queue<P, vector<P>, greater<P> > pque;
	P p = make_pair(0, 0);
	pque.push(p);
	int res = 0;
	while (!pque.empty())
	{
		P p = pque.top();
		pque.pop();
		int v = p.second;
		if (isUsed[v])
		{
			continue;
		}
		isUsed[v] = true;
		res += p.first;
		for (int i = 0; i < G[v].size(); ++i)
		{
			edge2 e = G[v][i];
			pque.push(P(e.cost, e.to));
		}
	}
	return(res);
}


void solve()
{
	cin >> n;

// For Kruskal
	//vector<edge> es;
	//for (int i = 0; i < n; ++i)
	//{
	//	for (int j = 0; j < n; ++j)
	//	{
	//		int cost;
	//		cin >> cost;
	//		if (cost != -1)
	//		{
	//			edge e;
	//			e.u = i;
	//			e.v = j;
	//			e.cost = cost;
	//			es.push_back(e);
	//		}
	//	}
	//}
	//cout << kruskal(es) << endl;

// For Prim
	vector<edge2> G[SIZE];
	for (int i = 0; i < n; ++i)
	{
		for (int j = 0; j < n; ++j)
		{
			int cost;
			cin >> cost;
			if (j >= i && cost != -1)
			{
				edge2 e1, e2;
				e1.to = j;
				e1.cost = cost;
				e2.to = i;
				e2.cost = cost;
				G[i].push_back(e1);
				G[j].push_back(e2);
			}
		}
	}
	cout << prim(G) << endl;
}

int main()
{
	solve();
	return(0);
}

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

int main()
{
    int n,tmp,sum=0,MIN;
    int d[105],v[105],m[105][105],p,child;
    cin>>n;
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++)cin>>tmp,m[i][j]=tmp;
        v[i]=0;
    }
    vector<int>G;
    v[0]=1;
    G.push_back(0);
    for(int M=1;M<n;M++)
    {
        MIN=2005;
        for(int i=0;i<M;i++)
            for(int j=0;j<n;j++)
                if(m[G[i]][j]!=-1&&!v[j]&&m[G[i]][j]<MIN)
                {
                    MIN=m[G[i]][j];
                    p=G[i];
                    child=j;
                }
        G.push_back(child);
        sum+=m[p][child];
        v[child]=1;
    }
    cout<<sum<<endl;
    return 0;
}

C++: #include <iostream>
#include <vector>
#include <list>
#include <algorithm>
#include <queue>
#include <stack>
#include <math.h>

#define ll long long
#define Int int
#define loop(x, start, end) for(Int x = start; x < end; x++)
#define loopdown(x, start, end) for(int x = start; x > end; x--)

using namespace std;

#define MAX_N 100
#define MAX_INT (1 << 21)


Int prim(Int W[MAX_N][MAX_N], Int n) {
  Int parent[n], key[n];
  bool mst_set[n];
  
  loop(i,0,n) key[i] = MAX_INT, mst_set[i] = false;
  
  key[0] = 0, parent[0] = -1;
  
  // why n-1 ?
  loop(count,0,n-1) {
      Int u = -1;
      Int min_ = MAX_INT;
      loop(v,0,n) {
          if (mst_set[v] == false && key[v] < min_) {
              min_ = key[v], u = v;
          }
      }
      
      mst_set[u] = true;
      
      loop(v,0,n) {
          if (W[u][v] != MAX_INT && mst_set[v] == false && W[u][v] < key[v]) {
              parent[v] = u, key[v] = W[u][v];
          }
      }
  }

  Int sum = 0;
  loop(v,1,n) {
    sum += W[v][parent[v]];
  }

  return sum;
}


int main(void) {
  Int n, w;
  Int W[MAX_N][MAX_N];
  cin >> n;
  loop(x,0,n) {
    loop(y,0,n) {
      cin >> w;
      W[x][y] = W[y][x] = (w == -1) ? MAX_INT : w;
    }
  }

  cout << prim(W, n) << endl;
}