Go: package main

import (
	"fmt"
	"sort"
)

type Edge struct {
	I int
	J int
	A int
}

func main() {
	var n int
	fmt.Scan(&n)

	g := make([][]int, n)
	edges := make([]Edge, 0, n*(n-1)/2)
	parents := make([]int, n)
	for i := 0; i < n; i++ {
		g[i] = make([]int, n)
		for j := 0; j < n; j++ {
			fmt.Scan(&g[i][j])
			if g[i][j] >= 0 {
				edges = append(edges, Edge{i, j, g[i][j]})
			}
		}
		parents[i] = i
	}
	sort.SliceStable(edges, func(i, j int) bool { return edges[i].A >= edges[j].A })

	sum := 0
	num := 0
	for len(edges) > 0 && num < n-1 {
		edge := edges[len(edges)-1]
		edges = edges[:len(edges)-1]

		if same(edge.I, edge.J, &parents) {
			continue
		}
		union(edge.I, edge.J, &parents)
		sum += edge.A
		num += 1
	}
	fmt.Println(sum)
}



func root(id int, parents *[]int) int {
	if id == (*parents)[id] {
		return id
	}
	return root((*parents)[id], parents)
}

func union(i, j int, parents *[]int) {
	(*parents)[root(j, parents)] = (*parents)[root(i, parents)]
}

func same(i, j int, parents *[]int) bool {
	return root(i, parents) == root(j, parents)
}

C++: #include <iostream>
#include <cstring>
#define INF 2000
using namespace std;

int graph[105][105],n,closest[105],color[105],p[105];
int sum=0;
void prim(){
    int i,j;
    while(1){
        int v=-1,minv=INF;
        for(i=0;i<n;i++){
            if(minv>closest[i]&&color[i]!=1){
                minv=closest[i];
                v=i;
            }
        }
        if(v==-1)break;
        color[v]=1;
        closest[v]=INF;
        for(i=0;i<n;i++){
            if(color[i]!=1&&graph[v][i]<closest[i]&&graph[v][i]!=-1){
                closest[i]=graph[v][i];
                p[i]=v;
            }
        }
    }

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

int main()
{
    memset(color,0,sizeof(color));
    memset(graph,-1,sizeof(graph));
    cin>>n;
    int i,j;
    for(i=0;i<n;i++){
        for(j=0;j<n;j++){
            cin>>graph[i][j];
        }
    }
    for(i=0;i<n;i++){
        p[i]=-1;
        closest[i]=INF;
    }
    closest[0]=0;
    prim();
    cout<<sum<<endl;
    return 0;
}

C++: #define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES
//#include "MyMath.h"
//#include "MyDisjointset.h"
#include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <functional>
#include <stdio.h>
using namespace std;
typedef pair<int, int> P;
const int INF = 2000000000;
int main() {
	int n; cin >> n;
	int m[101][101];
	for (int i = 0; i < n; i++)for (int j = 0; j < n; j++)
		cin >> m[i][j];
	int res = 0;
	int *d = new int[n];
	fill(d, d + n, INF); 
	d[0] = 0;
	bool *visit = new bool [n];
	fill(visit, visit + n, false);
	priority_queue <P, vector<P>, greater<P> > q;
	q.push(P(d[0], 0));
	while (!q.empty()) {
		int f = q.top().first, s = q.top().second;
		visit[s] = true;
		res += f;
		q.pop();
		for (int i = 0; i < n; i++) {
			if (!visit[i]) {
				if (m[s][i] != -1) {
					if (m[s][i] < d[i]) {
						d[i] = m[s][i];
						q.push(P(d[i], i));
					}
				}
				}
			}
		}
	cout << res << endl;
	delete[] d, visit;
	return 0;

C++: #include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
typedef pair<int, int> P;

// Kruskal法
//   証明: http://drken1215.hatenablog.com/entry/20121223/1356230697

class UnionFind {
    vector<int> par;
    int n;
public:
    UnionFind(int sz) : par(sz, -1) {}
    int find(int x) {
        if (par[x] < 0) return x;
        return par[x] = find(par[x]);
    }
    bool unite(int x, int y) {
        x = find(x); y = find(y);
        if (x == y) return false;
        if (par[x] > par[y]) swap(x, y);
        par[x] += par[y];
        par[y] = x;
        return true;
    }
    bool same(int x, int y) { return find(x) == find(y); }
    int size(int x) { return -par[find(x)]; }
};

int kruskal(vector<vector<P>> &edge) {
    int n = edge.size();
    typedef pair<int, P> PP;
    vector<PP> weights;
    for (int i = 0; i < n; i++) {
        for (auto &e : edge[i]) weights.push_back({e.second, {i, e.first}});
    }
    sort(weights.begin(), weights.end());
    UnionFind uf(n);
    int ret = 0;
    for (auto &w : weights) {
        int from = w.second.first;
        int to = w.second.second;
        if (uf.unite(from, to)) ret += w.first;
    }
    return ret;
};


// verified
//   http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=GRL_2_A&lang=jp
void AOJ_GRL_2_A() {
    int V, E;
    cin >> V >> E;
    vector<vector<P>> edge(V);
    for (int i = 0; i < E; i++) {
        int s, t, w;
        cin >> s >> t >> w;
        edge[s].push_back({t, w});
    }
    cout << kruskal(edge) << endl;
}

// verified
//   http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ALDS1_12_A&lang=jp
void AOJ_ALDS1_12_A() {
    int n;
    cin >> n;
    vector<vector<P>> edge(n);
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            int w;
            cin >> w;
            if (w < 0) continue;
            if (j < i) continue;
            edge[i].push_back({j, w});
        }
    }
    cout << kruskal(edge) << endl;
}

int main() {
    // AOJ_GRL_2_A();
    AOJ_ALDS1_12_A();
    return 0;
}

C++: #include <iostream>
#define N 100
#define WHITE 0
#define BLACK 1
#define INF 10000

using namespace std;

int M[N][N],n,d[N];

void prim(){
  int color[n],p[n];
  for(int i=0;i<n;i++){
    color[i]=WHITE;
    d[i]=INF;
  }
  d[0]=0;
  p[0]=-1;
  while(1){
    int mincost=INF;
    int u;
    for(int i=0;i<n;i++){
      if(color[i]!=BLACK && d[i]<mincost){
        mincost=d[i];
        u=i;
      }
    }
    if(mincost==INF) break;
    //connect u to p[u]
    color[u]=BLACK;
    for(int v=0;v<n;v++){
      if(color[v]!=BLACK && M[u][v]!=-1){
        if(M[u][v]<d[v]){
          d[v]=M[u][v];
          p[v]=u;
        }
      }
    }
  }
}

int main(){
  cin >> n;
  for(int i=0;i<n;i++){
    for(int j=0;j<n;j++) cin >> M[i][j];
  }
  prim();
  int sum=0;
  for(int i=0;i<n;i++) sum+=d[i];
  cout << sum << endl;
}

C++: #if 0
????????£????????????1.5h
??¢?????????????????§??????????????¢??°????????£????????????sum????¨??????¨????????????????????£????????§?????????
???????????¢??????????????§????????????????????????????????????"-1"???????????????????????????????????§?°???????????????????
#endif

#include <iostream>
#include <algorithm>
#include <utility>
using namespace std;

int P[10000+10];

void init(int N){
	for(int i=0;i<=N;++i) P[i]=i;
}
int root(int a){
	if(P[a]==a)return a;
	else return(P[a]=root(P[a]));
}
bool is_same_set(int a,int b){
	return root(a)==root(b);
}
void unite(int a,int b){
	P[root(a)]=root(b);
}
int main(){
	int N;
	cin>>N;
	int A[N][N];
	
	for(int i=0; i<N;i++){
		for(int j=0;j<N;j++){
			cin>>A[j][i]; //???????????\???
		}
	}

	int M = 0; 
	int cost[N*N],src[N*N],dst[N*N];
	for (int r=0; r<N;r++)
		for (int c=0; c<r;c++) //r<c???????§?????????????
			if (A[r][c] > -1) {
				cost[M] = A[r][c];
				src[M] = r;
				dst[M] = c;
				M += 1;
			}
	pair<int,int> edges[100000+10];
	for(int i=0;i<M;i++)
		edges[i]=make_pair(cost[i],i); //first???????????????second???ID
	sort(edges,edges+M);
	
	//for(int i=0;i<M;++i)
		//cout<<edges[i].first<<endl;

	init(N);
	int sum=0;
	for(int i=0;i<M;i++){
		if(is_same_set(src[edges[i].second],dst[edges[i].second])==true){
			unite(src[edges[i].second],dst[edges[i].second]);
			sum+=edges[i].first;
		}
	}
		cout<<sum<<endl;
}