Go: package main

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

const (
	MAX_N      = 200
	MAX_WEIGHT = 2000
)

type Edge struct {
	start  int
	end    int
	weight int
}

type ShortestInfo struct {
	edges    []Edge
	shortest int
}

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)

	n, err := nextInt(sc)
	if err != nil {
		panic(err)
	}

	shortestList := make([]ShortestInfo, n)
	for i := 1; i < n; i++ {
		shortestList[i] = ShortestInfo{edges: []Edge{}, shortest: MAX_N*MAX_WEIGHT + 1}
	}

	adjacentMatrix := make([][]int, n)
	for i := 0; i < n; i++ {
		adjacentMatrix[i] = make([]int, n)
		for j := 0; j < n; j++ {
			v, err := nextInt(sc)
			if err != nil {
				panic(err)
			}
			adjacentMatrix[i][j] = v
		}
	}

	mst([]int{1}, shortestList, adjacentMatrix)
	fmt.Println(calculateMinWeight(shortestList))
}

func calculateMinWeight(shortestList []ShortestInfo) int {
	visitedList := make([]bool, len(shortestList))
	visitedList[0] = true
	sumWeight := 0
	for !visitedAll(visitedList) {
		shortestInfo := shortestList[0]
		shortestList = shortestList[1:]
		edges := shortestInfo.edges
		for _, edge := range edges {
			if !visitedList[edge.end-1] {
				visitedList[edge.end-1] = true
				sumWeight += edge.weight
			}
		}
	}
	return sumWeight
}

func visitedAll(visitedList []bool) bool {
	for _, v := range visitedList {
		if !v {
			return false
		}
	}
	return true
}

func mst(nodeStack []int, shortestList []ShortestInfo, adjacentMatrix [][]int) {
	for len(nodeStack) > 0 {
		currentNode := nodeStack[0]
		nodeStack = nodeStack[1:]
		currentShortest := getShortest(currentNode, shortestList)
		adjacents := findAdjacents(currentNode, adjacentMatrix)
		for i := 0; i < len(adjacents); i++ {
			adj := adjacents[i]
			s := getShortest(adj.end, shortestList)
			if currentShortest.shortest+adj.weight < s.shortest || (currentShortest.shortest+adj.weight == s.shortest && len(currentShortest.edges)+1 > len(s.edges)) {
				setShortest(adj.end, adj, currentShortest, shortestList)
				nodeStack = append(nodeStack, adj.end)
			}
		}
	}
}

func getShortest(node int, shortestList []ShortestInfo) ShortestInfo {
	return shortestList[node-1]
}

func setShortest(node int, edge Edge, currentShortest ShortestInfo, shortestList []ShortestInfo) {
	newEdges := make([]Edge, len(currentShortest.edges)+1)
	for i := 0; i < len(newEdges)-1; i++ {
		newEdges[i] = currentShortest.edges[i]
	}
	newEdges[len(newEdges)-1] = edge
	shortestInfo := ShortestInfo{edges: newEdges, shortest: currentShortest.shortest + edge.weight}
	shortestList[node-1] = shortestInfo
}

func findAdjacents(currentNode int, adjacentMatrix [][]int) []Edge {
	adjacents := adjacentMatrix[currentNode-1]
	edges := []Edge{}
	for i, v := range adjacents {
		if v > 0 {
			edges = append(edges, Edge{start: currentNode, end: i + 1, weight: v})
		}
	}
	return edges
}

func nextInt(sc *bufio.Scanner) (int, error) {
	sc.Scan()
	return strconv.Atoi(sc.Text())
}

C++: #include <iostream>
#include <vector>
#include <queue>
#include <tuple>

int main(int argc, char const *argv[]) {
  int n;
  std::vector<std::vector<int> > a;
  std::cin >> n; // .. 100
  a = std::vector<std::vector<int> >(n, std::vector<int>(n));
  // a_ij <= 2000, a_ij = a_ji, this graph is connected;

  for (size_t i = 0; i < n; i++) {
    for (size_t j = 0; j < n; j++) {
      std::cin >> a[i][j];
    }
  }

  //ALGORITHM
  std::vector<bool> found_vtx(n, false);
  // std::tuple<int,int,int> x(1,2,3);
  std::priority_queue<
    std::tuple<int,int,int>,
    std::vector<std::tuple<int, int, int> >,
    std::greater<std::tuple<int, int, int> >
  > q;

  int new_elm = 0;
  found_vtx[new_elm] = true;
  for (size_t i = 0; i < n; i++) {
    q.push(std::make_tuple(a[new_elm][i], new_elm, i));
  }
  int mst_weight = 0;
  while(!q.empty()){
    auto x = q.top(); q.pop();
    // std::cout << "e: "
    //   << std::get<0>(x) << " "
    //   << std::get<1>(x) << " "
    //   << std::get<2>(x) << std::endl;
    if(found_vtx[std::get<2>(x)] || std::get<0>(x) < 0) continue;
    else{
      mst_weight += std::get<0>(x);
      // std::cout << std::get<0>(x) << std::endl;
      new_elm = std::get<2>(x);
      found_vtx[new_elm] = true;
      for (size_t i = 0; i < n; i++) {
        q.push(std::make_tuple(a[new_elm][i], new_elm, i));
      }
    }
  }

  std::cout << mst_weight << std::endl;
  return 0;
}

C++: #include <stdio.h>

#define N 2000
#define BLACK 0
#define WHITE 1
#define GRAY 2
#define inf 200000

int Map[N][N];
int n;
int color[N];
int d[N];
int p[N];


void init(){
	int i;
	for(i=0;i<n;i++){
		color[i]=WHITE;
		d[i] = inf;
	}
}

void prim(){
	init();
	int mincost;
	d[0] = 0;
	p[0] = -1;
	int i;
	int u;
	int v;
	while(1){
		mincost = inf;
		for(i=0;i<n;i++){
			if(color[i]!=BLACK && d[i] < mincost){
				mincost = d[i];
				u = i;
			}
		}

		if(mincost == inf){
			break;
		}
		color[u]=BLACK;
		for(v=0;v<n;v++){
			if(color[v] != BLACK && Map[u][v]!= -1){
				if(Map[u][v] < d[v]){
					d[v] = Map[u][v];
					color[v] = GRAY;
					p[v] = u;
				}
			}
		}
	}
}

int main(){
	scanf("%d",&n);
	int i,j;
	for(i=0;i<n;i++){
		for(j=0;j<n;j++){
			scanf("%d",&Map[i][j]);
		}
	}
	prim();
	int total = 0;
	for(i=0;i<n;i++){
		total += d[i];
	}
	printf("%d\n",total);
	return 0;
}

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

#define V vector<vector<pair<int,int>>>

int main() {
	cin.tie(0); ios::sync_with_stdio(false);
	int n,Ans = 0; cin >> n;
	V v(2001);
	vector<int> f(n + 1,0);
	for (int i = 1; i <= n;i++) {
		for (int j = 1; j <= n; j++) {
			int a; cin >> a;
			if (a != -1) {
				v[a].push_back(make_pair(i, j));
			}
		}
	}

	for (int i = 0; i <= 2000;i++) {
		for (int j = 0; j < v[i].size(); j++) {
			int first = v[i][j].first, second = v[i][j].second;
			if (f[first] >= 2 || f[second] >= 2) continue;
			Ans += i;
			f[first]++; f[second]++;
		}
	}

	cout << Ans << endl;
}