Go: package main

import (
	"bufio"
	"container/heap"
	"fmt"
	"os"
	"strconv"
)

type Node struct {
	id     int
	weight int
}

func init() {
	sc.Split(bufio.ScanWords)
}

type NodeHeap []Node

func (h NodeHeap) Len() int           { return len(h) }
func (h NodeHeap) Less(i, j int) bool { return h[i].weight < h[j].weight }
func (h NodeHeap) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }

func (h *NodeHeap) Push(x interface{}) {
	*h = append(*h, x.(Node))
}

func (h *NodeHeap) Pop() interface{} {
	old := *h
	n := len(old)
	x := old[n-1]
	*h = old[0 : n-1]
	return x
}

var sc = bufio.NewScanner(os.Stdin)

func scanInt() int {
	num, _ := strconv.Atoi(scanString())
	return num
}

func scanString() string {
	sc.Scan()
	return sc.Text()
}

func init() {
	sc.Split(bufio.ScanWords)
}

func main() {
	n := scanInt()

	list := make([][]int, n)
	seen := make([]bool, n)
	for i := range list {
		list[i] = make([]int, n)
		for j := range list[i] {
			list[i][j] = scanInt()
		}
	}

	start := Node{id: 0, weight: 0}
	h := &NodeHeap{start}
	heap.Init(h)

	sum := 0
	for h.Len() > 0 {
		node := heap.Pop(h).(Node)
		if seen[node.id] {
			continue
		}
		seen[node.id] = true
		sum += node.weight

		for i, w := range list[node.id] {
			if w < 0 {
				continue
			}
			heap.Push(h, Node{id: i, weight: w})
		}
	}

	fmt.Println(sum)
}

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>
#include <numeric>
#define REP(i, s, n) for (int i = s; i < n; ++i)
#define rep(i, n) REP(i, 0, n)
#define SORT(c) sort((c).begin(), (c).end())
#define SORT_INV(c) sort((c).begin(), (c).end(), greater<int>())
#define IINF INT_MAX
#define LLINF LLONG_MAX
#define DEBUG true
#define LL long long
#define Num 1000000007
// sort(a.begin(), a.end(), std::greater<int>());
using namespace std;

int main()
{
    int n;
    cin >> n;
    int a[n][n] = {0};
    rep(i, n)
    {
        rep(j, n)
        {
            int temp;
            cin>>temp;
            if(temp==-1) temp=INT_MAX;
            a[i][j]=temp;
        }
    }
    int mincost[n];
    bool used[n];
    rep(i, n)
    {
        mincost[i] = INT_MAX;
        used[i] = false;
    }
    //solve by prim
    mincost[0] = 0;
    int res = 0;
    while (true)
    {
        int v = -1;
        rep(i, n)
        {
            if (used[i] == false && (v == -1 || mincost[i] < mincost[v]))
                v = i;
        }
        if (v == -1)
            break;
        used[v] = true;
        res += mincost[v];
        rep(i, n)
        {
            mincost[i] = min(mincost[i], a[v][i]);
        }
    }cout<<res<<endl;
}

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

using namespace std;

#define Vi vector<int>
#define FOR(i,s,e) for (int i=s; i<e; i++)
#define ITER(v) v.begin(), v.end()

constexpr int N_MAX = 100;

int weight[N_MAX][N_MAX];

Vi mst(const int n) {
    bool isrouted[n] = {false};
    Vi route;
    route.push_back(0);
    isrouted[0] = true;
    FOR(t,1,n) {
        int minval = -1;
        int nextnode = -1;
        for (const int i : route)
        FOR(j,0,n) {
            if (isrouted[j]) { continue; }
            const int w = weight[i][j];
            if (w < 0) { continue; }
            if (minval < 0 || minval > w) {
                minval = w;
                nextnode = j;
            }
        }
        route.push_back(nextnode);
        isrouted[nextnode] = true;
    }
    return move(route);
}

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

    int n;
    cin >> n;

    FOR(i,0,n) {
        FOR(j,0,n) {
            cin >> weight[i][j];
        }
    }

    Vi route = mst(n);
    int cost = 0;
    for (auto iter = route.begin(); iter != route.end() - 1; ++iter) {
        cost += weight[*iter][*(iter+1)];
    }
    cout << cost << endl;

    return 0;
}