Go: package main

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

const (
	MAX = 100
	INF = 1 << 21
	WHITE = 0
	GRAY = 1
	BLACK = 2
)

var (
	n int
	M [MAX][MAX]int
)

func prim() int {
	var u, minv int
	var d [MAX]int
	var p [MAX]int
	var color [MAX]int

	for i := 0; i < n; i++ {
		d[i] = INF
		p[i] = -1
		color[i] = WHITE
	}
	d[0] = 0
	for {
		minv = INF
		u = -1
		for i := 0; i < n; i++ {
			if minv > d[i] && color[i] != BLACK {
				u = i
				minv = d[i]
			}
		}
		if u == -1 {
			break
		}
		color[u] = BLACK
		for v := 0; v < n; v++ {
			if color[v] != BLACK && M[u][v] != INF {
				if d[v] > M[u][v] {
					d[v] = M[u][v]
					p[v] = u
					color[v] = GRAY
				}
			}
		}
	}
	sum := 0
	for i := 0; i < n; i++ {
		if p[i] != -1 {
			sum += M[i][p[i]]
		}
	}
	return sum
}

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	scanner.Split(bufio.ScanWords)
	scanner.Scan()
	n, _ = strconv.Atoi(scanner.Text())
	for i := 0; i < n; i++ {
		for j := 0; j < n; j++ {
			scanner.Scan()
			e, _ := strconv.Atoi(scanner.Text())
			if e == -1 {
				M[i][j] = INF
			} else {
				M[i][j] = e
			}
		}
	}
	fmt.Println(prim())
}

C++: #include <iostream>
using namespace std;
#define INF 100000000

int main(){
  long n;
  long G[100][100],d[100];
  long minc,mi,i;
  int flg[i];
  long sum=0;
  
  cin >> n;
  for (i=0;i<n;i++){
    for (int j=0;j<n;j++){
      cin >> G[i][j];
      if (G[i][j]==-1) G[i][j]=INF;
    }
    d[i]=INF;
    flg[i]=0;
  }

  d[0]=0;
  while (true){
    minc=INF;
    for (i=0;i<n;i++){
      if (!flg[i]&&d[i]<minc){
	minc=d[i];
	mi=i;
      }
    }
    if (minc==INF) break;
    flg[mi]=1;
    for (i=0;i<n;i++){
      if (G[mi][i]==INF)continue;
      if (!flg[i]&&d[i]>G[mi][i]){
	d[i]=G[mi][i];
	//cout << d[i];
      }
    }
  }
  
  for (i=0;i<n;i++){
    sum+=d[i];
  }
  cout << sum<<endl;
  
  return 0;
}

C++: #include <iostream>
using namespace std;
#define INF 1000000

int main(){
  long n;
  long G[100][100],d[100];
  long minc,mi,i;
  bool flg[i];
  long sum=0;
  
  cin >> n;
  for (i=0;i<n;i++){
    for (int j=0;j<n;j++){
      cin >> G[i][j];
      if (G[i][j]==-1) G[i][j]=INF;
    }
    d[i]=INF;
    flg[i]=false;
  }

  d[0]=0;
  while (true){
    minc=INF;
    for (i=0;i<n;i++){
      if (!flg[i]&&d[i]<minc){
	minc=d[i];
	mi=i;
      }
    }
    if (minc==INF) break;
    flg[mi]=true;
    for (i=0;i<n;i++){
      if (G[mi][i]==INF)continue;
      if (!flg[i]&&d[i]>G[mi][i]){
	d[i]=G[mi][i];
	//cout << d[i];
      }
    }
  }
  
  for (i=0;i<n;i++){
    sum+=d[i];
  }
  cout << sum<<endl;
  
  return 0;
}

C++: #include<iostream>
#include<cstdio>
using namespace std;
static const int MAX = 100;
static const int INFTY = (1 << 21);
static const int WHITE = 0;
static const int GRAY = 1;
static const int BLACK = 2;

int n, M[MAX][MAX];

int prim() {
	int u, minv;
	int d[MAX], p[MAX], color[MAX];

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

	d[0] = 0;

	while (1) {
		minv = INFTY;
		u = -1;
		for (int i = 0; i < n; i++) {
			if (minv > d[i] && color[i] != BLACK) {
				u = i;
				minv = d[i];
			}
		}
		if (u == -1)break;
		color[u] = BLACK;
		for (int v = 0; v < n; v++) {
			if (color[v] != BLACK && M[u][v] != INFTY) {
				if (d[v] > M[u][v]) {
					d[v] = M[u][v];
					p[v] = u;
					color[v] = GRAY;
				}
			}
		}
	}

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

int main(void) {
	scanf("%d", &n);

	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			int e; cin >> e;
			M[i][j] = (e == -1) ? INFTY : e;
		}
	}
	int k = prim();
	printf("%d\n", k);
	return 0;
}

C++: #include <algorithm>
#include <cassert>
const int MaxNode = 100;

struct UnionFind {
    int node_num;
    int parent[MaxNode];
    int weight = 0;
    
    UnionFind(int node_num) : node_num(node_num) {
        std::fill(parent, parent + node_num, -1);
    }
    
    int root(int u) {
        return parent[u] < 0 ? u : parent[u] = root(parent[u]);
    }
    
    bool find(int u, int v) {
        return root(u) == root(v);
    }
    
    bool unite(int u, int v, int cost) {
        if ((u = root(u)) == (v = root(v))) {
            return false;
        } else {
            if(parent[u] > parent[v]) {
                std::swap(u, v);
            }
            parent[u] += parent[v];
            parent[v] = u;
            weight += cost;
            return true;
        }
    }
};
template <typename T>
struct Edge {
    
    Edge() = default;
    Edge(int from, int to, int cost) : from(from), to(to), cost(cost) {}
    
    Edge(const Edge&) = default;
    Edge& operator=(const Edge&) = default;
    Edge& operator=(Edge&&) = default;
    
    
    int from;
    int to;
    T cost;
};

#include <iostream>
using namespace std;

int main() {
	int n;
	cin >> n;
	UnionFind uf(n);
	
	std::vector<Edge<int>> edges(n);
	
	for(int i = 0; i < n; ++i) {
		for(int j = 0; j < n; ++j) {
			int dist;
			cin >> dist;
			
			if(i != j && dist != -1) {
				edges.emplace_back(i, j, dist);
			}
		}
	}
	
	std::sort(edges.begin(), edges.end(), [](Edge<int> a, Edge<int> b){return a.cost < b.cost;});

	for (auto& e : edges) {
        if (!uf.find(e.from, e.to)) {
            uf.unite(e.from, e.to, e.cost);
        }
    }
    
    cout << uf.weight << endl;
	return 0;
}

C++: 
class UnionFind{
    int N;
    vector<int> par;
    vector<int> rank;
    public:
    UnionFind(int N):N(N),par(N),rank(N,0){iota(par.begin(),par.end(),0);}
    int find(int x);
    void unite(int x,int y);
    bool same(int x,int y){return find(x)==find(y);}
};

int UnionFind::find(int x){
    if(par[x]==x) return x;
    return par[x]=find(par[x]);
}
void UnionFind::unite(int x,int y){
    x=find(x),y=find(y);
    if(x==y) return;
    if(rank[x]>=rank[y]){
        if(rank[x]==rank[y]) rank[x]++;
        par[y]=x;
    }else{
        par[x]=y;
    }


}

#include<bits/stdc++.h>
#include"UnionFind.hpp"
using namespace std;

using Weight=long long;
struct Edge{
    int from,to;
    Weight weight;
};
bool operator<(const Edge& lhs,const Edge& rhs){
    return lhs.weight<rhs.weight;
}

using Edges=vector<Edge>;
using Graph=vector<Edges>;


Edges MinimumSpanningTree(int n,Edges edges){
    Edges res;
    UnionFind u(n);
    for(int i=0;i<edges.size();i++){
        if(!u.same(edges[i].from,edges[i].to)){
            res.push_back(edges[i]);
            u.unite(edges[i].from,edges[i].to);
        }
    }
    return res;
}

Edges MinimumSpanningTree(const Graph& g){
    Edges edges;
    for(int i=0;i<g.size();i++){
        copy(g[i].begin(),g[i].end(),back_inserter(edges));
    }
    auto res=MinimumSpanningTree(g.size(),edges);
    return res;
}

int main(){
    int n;
    cin>>n;
    Graph g(n);
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            Weight c;
            cin>>c;
            if(c==-1) continue;
            g[i].push_back({i,j,c});
        }
    }
    auto mst=MinimumSpanningTree(g);
    cout<<accumulate(mst.begin(),mst.end(),Weight(0),[](const Weight& lhs,const Edge& rhs){return lhs+rhs.weight;})<<endl;
    return 0;
}