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<iostream>
#include<stdio.h>
using namespace std;
static const int INFTY = (1 << 21);

int n, m[100][100];

int prim() {
    int u, minv;
    int d[100], p[100], color[100];
    
    for (int i = 0; i < n; i++) {
        d[i] = INFTY;
        p[i] = -1;
        color[i] = 0;
    }
    
    d[0] = 0;
    
    while(1) {
        minv = INFTY;
        u = -1;
        for (int i = 0; i < n; i++) {
            if (minv > d[i] && color[i] != 2) {
                u = i;
                minv = d[i];
            }
        }
        if (u == -1) break;
        color[u] = 2;
        for (int v = 0; v < n; v++) {
            if (color[v] != 2 && m[v][u] != INFTY) {
                if (d[v] > m[u][v]) {
                    d[v] = m[u][v];
                    p[v] = u;
                    color[v] = 1;
                }
            }
        }
    }
    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;
            if (e == -1) {
                m[i][j] = INFTY;
            } else {
                m[i][j] = e;
            }
        }
    }
    
    cout << prim() << endl;
    
    return 0;
}

C++: #include<iostream>
#include<vector>
#include<queue>
using namespace std;
template< typename FirstType, typename SecondType >
struct PairComparator {
  bool operator()( const pair<FirstType, SecondType>& p1, const pair<FirstType, SecondType>& p2 ) const 
    {  
		return p1.first > p2.first;
    }
};
bool inMST(vector< std::pair<int,int> > & MST,int j) {
	for (int k=0;k<(int)MST.size();++k){
		if(j == MST[k].second)
			return true;
	}
	return false;
}
int main(){
	int n;
	cin >> n;
	vector<int> E(n*n);
	vector< std::pair<int,int> > MST;

	for(int k=0;k<n*n;++k) {
		cin >> E[k];
	}
	priority_queue<pair<int,pair<int,int> >,vector< pair<int,pair<int,int> > >,
		PairComparator<int,pair<int,int> > > r;

	//put Node 0
	int cost = 0;
	int i=0;
	MST.push_back(pair<int,int>(i,i));
	while((int)MST.size() < n) {
		int ii=i*n;
		for (int j=0;j<n;++j) {
			if (E[ii+j] > 0) {
				pair<int,int> add = std::make_pair(i,j);
				if (!inMST(MST,add.second)){
					r.push(std::pair<int,std::pair<int,int> >(E[ii+j],add));
				}
			}
		}
		pair<int,pair<int,int> > top;
		while (!r.empty()) {
			top = r.top();
			r.pop();
			if (!inMST(MST,top.second.second)){
				break;
			}
		}
		cost += top.first;
		MST.push_back(top.second);
		i = top.second.second;
	}
	cout << cost << endl;
}

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

#define REP(i, n) for(int i=0;i<(int)(n);i++)
#define ALL(x) (x).begin(),(x).end()
const int INF = 1e9;

using namespace std;
int n;
int res = 0;
struct node {
    int id;
    int parent;
};

node v[100];
vector<tuple<int, int, int>> cost;
vector<int> path;

int find(node x){
    if(x.parent == -1){
        REP(i, path.size())
            v[path[i]].parent = x.id;
        path.clear();
        return x.id;
    }
    else{
        path.push_back(x.id);
        return find(v[x.parent]);
    }
}

void marge(node x, node y){
    int x_id = find(x);
    int y_id = find(y);
    if(x_id == y_id)
        return;
    else
        v[y_id].parent = x_id;
    return;
}

bool judge(node x, node y){
    vector<int> xpath, ypath;
    return find(x) == find(y);
}

int main() {
    cin >> n;
    REP(i, n) REP(j, n) {
        int tmp;
        cin >> tmp;
        if(tmp != -1)
            cost.push_back(make_tuple(tmp, i, j));
    }
    REP(i, n){
        v[i].id = i;
        v[i].parent = -1;
    }
    sort(ALL(cost));
    REP(i, cost.size()){
        node from = v[get<1>(cost[i])];
        node to = v[get<2>(cost[i])];
        if(judge(from, to)){
            continue;
        }
        res += get<0>(cost[i]);
        marge(from, to);
    }

    cout << res << endl;
    return 0;
}

C++: #include<iostream>

using namespace std;

static const int MAX = 100;
static const int INFTY = (1 << 21);
static const int WHITE = 0, GRAY = 1, 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] = INFTY;
    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; v < n; v++){
      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;
}