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 <utility>
#include <algorithm>
#define MAX 150
#define rep(i,p) for(int i=0;i<p;i++)
#define rep1(i,p) for(int i=1;i<=p;i++)
using namespace std;
int n,temp,a,b,size,ra,rb;
int A[MAX];
int B[MAX];
int W[MAX];
int V[MAX];
pair<int,int> edge[MAX];
int root (int child) {
  int a = child;
  while (V[a] != a) {
    a = V[a];
  }
  return a;
}
int main(){
  size = 0;
  cin >> n;
  rep(i,n) {
    rep(j,n) {
      cin >> W[size];
      if (i <= j && W[size] != -1) { 
        A[size] = i;
        B[size] = j;        
        edge[size].first = W[size];
        edge[size].second = size;
        size++;
      }
    }
  }
  // cout << endl;
  rep(i,size) {
    cout << A[edge[i].second] << " " << B[edge[i].second]
         << " " << edge[i].first << endl;;
  }
  // sort by lenght of edge
  sort(edge, edge+size);
  
  int sum = 0;
  rep(i,n) V[i] = i;
  rep(i,size) {
    // if (edge[i].first == -1) continue;
    temp = edge[i].second;
    // a , b : vertex
    a = A[temp];
    b = B[temp];
    ra = root(a);
    rb = root(b);
    if (ra != rb) {
      // ?????????????????????
      sum += edge[i].first;
      V[ra] = rb;
      // if (ra != a && rb == b){     
      //   V[b] = ra;
      // } else if (ra == a && rb != b) {
      //   V[a] = rb;
      // } else if (ra == a && rb == b) {        
      //   V[a] = b;
      // } else if (ra != a && rb != b) {                
      //   V[ra] = rb;
      // }
    }
  }
  cout << sum << endl;
  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define REP(i,n) for(ll i=0; i<(n); i++)
#define REP2(i,x,n) for(ll i=x; i<(n); i++)
#define REP3(i,x,n) for(ll i=x; i>(n); i--)
#define ALL(n) begin(n),end(n)

#define SENTINEL 2000000000
#define NIL -1
struct cww{cww(){ios::sync_with_stdio(false);cin.tie(0);}}star;
const long long INF = numeric_limits<long long>::max();
static const int MAX = 100;
static const int WHITE = 0;
static const int GRAY = 1;
static const int BLACK = 2;
static const int INFTY = (1<<21);

int n,M[MAX][MAX];

//primのアルゴリズム
int prim(){
    int color[MAX],d[MAX],p[MAX];
    int minv;
    for (int u = 0; u < n; u++)
    {
        color[u] = WHITE;
        d[u] = INFTY;
        p[u] = -1;
    }
    d[0]=0; 
    while(1){
        minv = INFTY;
        int u= -1;
        for (int i = 0; i < n; i++)
        {
            if(color[i]!=BLACK&&d[i]<minv){
                minv = d[i];
                u = i;
            }
        }
        if (u == -1)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[i][p[i]];
        }
    }
    return sum;
}

int main(){
    cin >> n;
    REP(i,n){REP(j,n){
        int e;
        cin >> e;
        M[i][j] = (e ==-1)?INFTY:e;    
        }
    }
    cout << prim() << endl;

    return 0;
}

C++: #include <iostream>
using namespace std;
 
#define WHITE 0
#define GRAY 1
#define BLACK 2
#define INFTY 1000000
 
 
int n,M[100][100],color[100],d[100],p[100];
 
//プリム法
int prim(){
  int i,u;

  //初期化
  for( i = 0; i < 100; i++ ){ 
    color[i] = WHITE;
    d[i] = INFTY;
    p[i] = -1;   
  }
  
  d[0] = 0;
  
  
  while(1){
    u = -1;
    int mincost = INFTY;
     
    for( i = 0; i < n; i++ ){
      if( color[i] != BLACK && d[i] < mincost ){
    mincost = d[i];
    u = i;
      }
    }
    
    if( u == -1 ) break;
 
    color[u] = BLACK;
 
    //最小全域木の更新
    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;
    }
      }
    }
  }
  //最小全域木の辺の重みの総和をもとめる
  int sum = 0;
  for( i = 0; i < n; i++ ){
    if( p[i] != -1 ) sum+= M[i][p[i]];
  }
  return sum;
}
 
int main(){
  //入力
  cin >> n;
  for( int i = 0; i < n; i++ ){
    for( int j = 0; j < n; j++ ){
      cin >> M[i][j];
      if( M[i][j] == -1 ) M[i][j] = INFTY;
    }
  }
 
  cout << prim() << endl;
 
  return 0;
}

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

vector<vector<int>> v;
vector<int> d, color;

const int WHITE = 0;
const int GRAY = 1;
const int BLACK = 2;

int n, ans;

void prim(){
  priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>> q;
  q.push(make_pair(0, 0));
  d[0] = 0;

  while(!q.empty()){
    auto next = q.top(); q.pop();
    if(color[next.second] == BLACK) continue;

    color[next.second] = BLACK;
    ans += d[next.second];

    for(int i=0; i<n; ++i){
      if(i != next.second && v[next.second][i] != -1 && color[i] != BLACK && d[i] > v[next.second][i]){
        q.push(make_pair(v[next.second][i], i));
        d[i] = v[next.second][i];
        color[i] = GRAY;
      }
    }
  }
}

int main(){
  cin >> n;

  v.resize(n);
  d.resize(n);
  color.resize(n);

  for(int i=0; i<n; ++i){
    d[i] = numeric_limits<int>::max();
    v[i].resize(n);
    for(int l=0; l<n; ++l) cin >> v[i][l];
  }

  prim();

  cout << ans << endl;
}