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>
using namespace std;
const int maxx=110;
const int infinity=(1<<30);
 
int A[maxx][maxx];
bool flag[maxx];	//用flag来标记这个节点是不是在MST中，true是在T里，false是在V-T里；
 
int main (){
		int n;
		cin>>n;
		for(int i =1;i<=n;i++){
			for(int j=1;j<=n;j++){
				int e;cin>>e;
				A[i][j]=(e==-1)?infinity:e;
			}
		}
 
		for(int i=0;i<maxx;i++) flag[i]=false;
		int sum=0,minv,minj;
		flag[1]=true;
		while(1){
			minv=infinity;
			minj=-1;
			for(int i=1;i<=n;i++){
				if(flag[i]==true){//找到一个在T中的顶点
					for(int j=1;j<=n;j++){
						if(A[i][j]!=-1 && A[i][j]<minv && flag[j]==false) {//找到一个在V-T中的顶点
							minv=A[i][j];//找到权值的最小值minv
							minj=j;//找到权值的最小值对应的顶点，就是下一步要加入到MST中的顶点minj
						} 
					}
				}
			}
//			cout<<minj<<endl;
			if(minv==infinity || minj==-1) break;
			flag[minj]=true;
			sum+=minv;
		}
		
		cout<<sum<<endl;
		return 0;
		
}

C++: #include<iostream>
using namespace std;

const int MAX = 100;


int main() {
	int n;
	int graph[MAX][MAX];
	bool table[MAX] = { false };
	int sum = 0;

	cin >> n;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			int temp;
			cin >> temp;
			if (temp == -1)temp = INT_MAX;
			graph[i][j] = temp;
		}
	}

	table[0] = true;

	for (int i = 0; i < n - 1; i++) {
		int cost = INT_MAX;
		int m;

		for (int j = 0; j < n; j++) {
			if (table[j] == false)continue;

			for (int k = 0; k < n; k++) {
				if (graph[j][k] < cost && table[k] == false) {
					cost = graph[j][k];
					m = k;
				}
			}
		}

		sum += cost;
		table[m] = true;
	}

	cout << sum << endl;

	return 0;
}

C++: #include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <ctime>
#include <cstring>
#include <functional>
#include <iostream>
#include <iomanip>
#include <limits>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <regex>
#include <vector>

#define fix(n)          cout<<fixed<<setprecision(n)
#define rep(i,n)        for (int i = 0; i < (n); ++i)
#define all(a)          (a).begin(), (a).end()
#define sort(a)         sort(all(a))
#define uniq(a)         sort(a);(a).erase(unique(all(a)), (a).end())
#define reverse(a)      reverse(all(a))
#define ctos(c)         string(1, (c))
#define out(d)          cout << (d)
#define outl(d)         std::cout<<(d)<<"\n"
#define YES()           cout << "YES" << endl
#define NO()            cout << "NO" << endl
#define Yes()           cout << "Yes" << endl
#define No()            cout << "No" << endl
#define ceil(x, y)      ((x + y - 1) / (y))
#define debug(x)        cerr << #x << ": " << (x) << '\n'
#define debug2(x, y)    cerr << #x << ": " << (x) << ", " << #y << ": " << (y) << '\n'
#define debug3(x, y, z) cerr << #x << ": " << (x) << ", " << #y << ": " << (y) << ", " << #z << ": " << (z) << '\n'
#define dbg(v)          for (size_t _ = 0; _ < v.size(); ++_){ cerr << #v << "[" << _ << "] : " << v[_] << '\n'; }
#define pb              push_back
#define fst             first
#define int             long long
#define INF             __LONG_LONG_MAX__

using namespace std;
using ll = long long;
using ld = long double;
using P = pair<ll,ll>;

const ll MOD = 1000000007; // 10^9 + 7
const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};

void solve() {
  const ll MAX = 100;

  ll n; cin >> n;
  ll mat[MAX][MAX];
  rep(i, n)rep(j, n) cin >> mat[i][j];

  vector<bool> mst(n, false);
  ll ans = 0;
  mst[0] = true;
  priority_queue<tuple<ll, ll, ll>> pq;
  for (int i = 1; i < n; i++) {
    if (mat[0][i] != -1) pq.emplace(-mat[0][i], 0, i);
  }

  ll w, f, t;
  rep(i, n-1) {
    while(1) {
      tie(w, f, t) = pq.top(); pq.pop();
      if (mst[f] && !mst[t]) break;
    }
    // debug3(-w, f, t);
    ans += mat[f][t];
    mst[t] = true;

    for (int j = 0; j < n; j++) {
      if (t != j && mat[t][j] != -1) pq.emplace(-mat[t][j], t, j);
    }
  }
  outl(ans);
}

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  srand((unsigned)time(NULL));
  fix(12);

  solve();
}

C++: #include <iostream>
#include <vector>
#include <utility>
#include <algorithm>
using namespace std;
int n;
int A[110][110];
int P[10010] = {0};
void init(int N){
	for(int i=0;i<=N;++i) P[i] = i;
}

int root(int a){
	if(P[a] == a) return a;
	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(){
	while(cin >> n && n>0){
		init(n);
		for(int i=0;i<n;i++){
			for(int j=0;j<n;j++){
				cin >> A[i][j];
			}
		}
		int M = 0;
		int cost[10000],src[10000],dst[10000];
		vector< pair<int,int> > edges;
		for(int r=0;r<n;r++){
			for(int c=r+1;c<n;c++){
				if(A[r][c] > -1){
					cost[M] = A[r][c];
					src[M] = r;
					dst[M] = c;
					//cout << cost[M] << endl;
					pair<int,int> edge = make_pair(cost[M],M);
					edges.push_back(edge);
					M++;
				}
			}
		}
		sort(edges.begin(),edges.end());
		long long  sum = 0;
		for(int i=0;i<M;i++){
			int n1 = src[edges[i].second],n2 = dst[edges[i].second];
			if(!is_same_set(n1,n2)){
				unite(n1,n2);
				sum += edges[i].first;
			}
		}
		cout << sum << endl;
	}
}

#if 0
所要時間は45分程度。クラスカル法を回す部分の実装に少し苦戦した。
#endif