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<algorithm>
#include<cstdio>
#include<cmath>
#include<math.h>
#include<string>
#include<string.h>
#include<stack>
#include<queue>
#include<vector>
#include<utility>
#include<set>
#include<map>
#include<stdlib.h>
#include<iomanip>

using namespace std;

#define ll long long
#define ld long double
#define EPS 0.0000000001
#define INF 1e9
#define rep(i,n) for(i=0;i<n;i++)
#define loop(i,a,n) for(i=a;i<n;i++)
#define all(in) in.begin(),in.end()
#define shosu(x) fixed<<setprecision(x)

typedef vector<int> vi;
typedef pair<int,int> pii;


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

bool comp(const edge& e1, const edge& e2){

  return e1.cost < e2.cost;

}

#define MAX_N 100000
#define MAX_E 100000

int par[MAX_N];

int rank[MAX_N];

void init(int n){

  for(int i = 0 ; i < n ; i++){

    par[i] = i;

    rank[i] = 0;

  }

}

int find(int x){

  if(par[x] == x) return x;

  else return par[x] = find(par[x]);

}

void unite(int x, int y){

  x = find(x);

  y = find(y);

  if(x == y) return;

  if(rank[x] == rank[y]) par[x] = y;

  else par[y] = x;

  if(rank[x] == rank[y]) rank[x]++;
 
}

bool same(int x, int y){

  return find(x) == find(y);

}

edge es[MAX_E];

int V, E;

int kruskal(){

  sort(es, es+E, comp);

  init(V);

  int res = 0;

  for(int i = 0 ; i < E ; i++){

    edge e = es[i];

    if(!same(e.u, e.v)){

      unite(e.u, e.v);

      res += e.cost;

    }

  }

  return res;

}

int main(){
  int n;
  cin>>n;
  V=n;
  int i,j,k;
  E=0;
  rep(i,n)rep(j,n){
    int t;
    cin>>t;
    if(t<0)continue;
    es[E].u=i;
    es[E].v=j;
    es[E].cost=t;
    E++;
  }
  cout<<kruskal()<<endl;
}

C++: #include<iostream>
#include<utility>
#include<algorithm>
using namespace std;
int p[101];
void makeset(int N){
    for(int i=0;i<N;++i){
        p[i]=i;
    }
}
int root(int x){
    if(p[x]==x) return x;
    return (p[x]==root(p[x]));
}
void unite(int x,int y){
    p[root(x)]=root(y);
}
bool same(int x,int y){
    return root(x)==root(y);
}
int main(){
    int N,M=0;
    int A[101][101],cost[101],src[101],dst[101];
    cin>>N;
    makeset(N);
    for(int r=0;r<N;++r){
        for(int c=0;c<N;++c){
            cin>>A[r][c];
        }
        for(int c=r+1;c<N;++c){
            if(A[r][c]>-1){
                cost[M]=A[r][c];
                src[M]=r;
                dst[M]=c;
                M+=1;
            }
        }
    }

    
    pair<int,int>edges[101];
    for(int i=0;i<M;++i){
        edges[i]=make_pair(src[i],dst[i]);
    }
    sort(edges,edges+M);
    sort(cost,cost+M);
    int total=0,num=0,flag=0;
    for(int i=0;i<M;++i){
        unite(edges[i].first,edges[i].second);
        total+=cost[i];
        num++;
        if(num==N-1){
            flag=1;
            break;
        }
    }
    if(flag){
        cout<<total<<endl;
    }
}

C++: #include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using vi = vector<int>;    using vvi = vector<vi>;
using vb = vector<bool>;   using vvb = vector<vb>;
using vl = vector<ll>;     using vvl = vector<vl>;
using vd = vector<double>; using vvd = vector<vd>;

#define REP(i,n) for(auto i = 0 * n; i < (n); ++i)
#define ALL(c) (c).begin(), (c).end()
#define FOR(i,s,e) for (ll i = s; i < (ll)e; i++)
#define TEN(x) ((ll)1e##x)

class UnionFind {
	typedef size_t U;
	vector<int32_t> v;
public:
	UnionFind(U n) : v(n, -1) {}
	U find(U x) { return v[x] < 0 ? x : (v[x] = find((U)v[x])); }
	bool same(U x, U y) { return find(x) == find(y); }
	U size(U x) { return (U)-v[find(x)]; }
	void unite(U x, U y) {
		x = find(x); y = find(y);
		if (x != y) {
			if (-v[x] < -v[y]) swap(x, y);
			v[x] += v[y];
			v[y] = x;
		}
	}
};

struct Edge{ uint32_t to, id; };
typedef vector<vector<Edge>> Graph;

template<class T> struct Kruskal{
	typedef uint32_t U;
	template<class V> Kruskal(U n, const V & u, const V & v, const vector<T> & w) : g(n) {
		vector<U> es(w.size());
		iota(es.begin(), es.end(), 0);
		sort(es.begin(), es.end(), [&](U i, U j) { return w[i] < w[j]; });
		UnionFind uf(n);
		for (U i : es) {
			if (!uf.same(u[i], v[i])) {
				uf.unite(u[i], v[i]); cost += w[i];
				g[u[i]].push_back({ (U)v[i], i }); g[v[i]].push_back({ (U)u[i], i });
			}
		}
	}
	Graph g; T cost = 0;
};

int main() {
	cin.tie(0);
	ios_base::sync_with_stdio(false);
	cout << fixed << setprecision(50);

	ll n; cin >> n;
	vl u, v, w;
	REP(i, n) REP(j, n) {
		ll x; cin >> x;
		if (x != -1) {
			u.push_back(i);
			v.push_back(j);
			w.push_back(x);
		}
	}
	Kruskal<ll> mst(n, u, v, w);
	cout << mst.cost << endl;
}

C++: #include <iostream>

using namespace std;
static const int inf = 10000;
static const int emp = -1;
static const int black = 1;
static const int white = 0;

int   graph[124][124] = {{}};;
int d[124] = {};
int color[124] = {};
int p[124] = {};
int n;

int  prim(){

  for(int i = 0;i < n;i++){
    d[i] = inf;
    color[i] = white;
    p[i] = -1;
  }

  d[0] = 0;
  int u ,v;
 

  while(1){

    v = inf;
    u = -1;

    for(int i = 0;i < n;i++){
      if(d[i] < v && color[i] != black){
	v= d[i];
	u = i;
      }
    }
    //    cout << "u = " << u << endl; 

    if(u == -1) break;
    color[u] = black;

    // cout << 'a' << endl;
    for(int i = 0;i < n;i++){
      if(graph[u][i] != -1 && color[i] != black){
	if(graph[u][i] < d[i]){
	  d[i] = graph[u][i];
	  p[i]= u;
	  //  cout << "d = " << d[i] <<endl;
	  // cout << "i = " << i << endl;
	}
      }
    }
  }

  int sum = 0;
  for(int i = 0;i < n;i++){
    if(p[i] != -1) sum += graph[i][p[i]];
  }
  return sum;
}



int main(){

  cin >> n;

  for(int i = 0;i < n;i++){
    for(int j = 0;j < n;j++){
      cin >> graph[i][j];
    }
  }



  cout <<  prim() << endl;

  return 0;
}