Go: package main

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

//util
var sc = bufio.NewScanner(os.Stdin)

func nextStr() string {
	sc.Scan()
	return sc.Text()
}

func nextInt() int {
	sc.Scan()
	i, e := strconv.Atoi(sc.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func printList(list []int) {
	n := len(list)
	buf := bufio.NewWriter(os.Stdout)
	for _, v := range list[0 : n-1] {
		buf.Write([]byte(fmt.Sprintf("%d ", v)))
	}
	buf.Write([]byte(fmt.Sprintf("%d\n", list[n-1])))
	buf.Flush()
}

func max(a, b int) int {
	if a > b {
		return a
	} else {
		return b
	}
}

func min(a, b int) int {
	if a > b {
		return b
	} else {
		return a
	}
}

//solve
const (
	MAX   = 100
	INFTY = (1 << 21)
	WHITE = 0
	GRAY  = 1
	BLACK = 2
)

//V: グラフの頂点全体の集合
//T: MSTに属する頂点の集合
var (
	n     int
	M     [MAX][MAX]int //重みを記録する隣接行列
	color [MAX]int
	p     [MAX]int //MSTにおける頂点の親を記録
	d     [MAX]int //Tに属する頂点と、V−Tに属する頂点を繋ぐ辺の中で、重みが最小のものを記録
)

func prim() int {
	var u, minv int
	//初期化
	for i := 0; i < n; i++ {
		d[i] = INFTY
		p[i] = -1
		color[i] = WHITE
	}

	d[0] = 0

	for {
		//頂点uを決めていく
		//つまり、Tに属する頂点と、V−Tに属する頂点を繋ぐ辺の中で、重みが最小のものを決める
		minv = INFTY
		u = -1
		for i := 0; i < n; i++ {
			//未訪問かつ、辺の重みが最小となるuを選ぶ
			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++ {
			//未訪問かつ、辺が存在するとき
			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
				}
			}
		}
	}

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

func main() {
	sc.Split(bufio.ScanWords)
	n = nextInt()

	for i := 0; i < n; i++ {

		for j := 0; j < n; j++ {
			tmp := nextInt()
			if tmp == -1 {
				tmp = INFTY
			}
			M[i][j] = tmp
		}

	}

	fmt.Println(prim())

}

C++: #include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <utility>

using namespace std;

#define REP(var, count) for(int var=0; var<count; var++)

struct unionf {
	
	vector<int> data;
	
	unionf(int n) {
		REP(i, n+1) { data.push_back(i); }
	}
	
	int root(int a) {
		if(data[a] == a) { return a; }
		return (data[a] = root(data[a]));
	}
	
	bool issame(int a, int b) {
		return root(a) == root(b);
	}
	
	void unite(int a, int b) {
		data[root(a)] = root(b);
	}

};

int main(void) {
	int N;
	cin >> N;
	
	vector<pair<int, pair<int, int> > > edge;
	
	REP(x, N) {
		REP(y, N) {
			int c;
			cin >> c;
			if ( x <= y ) { continue; }
			if ( c == -1 ) { continue; }
			edge.push_back( make_pair(c, make_pair(x,y)) );
		}
	}
	
	sort(edge.begin(), edge.end());
	
	unionf uf(N);
	
	int cnt = 0;
	int ret = 0;
	REP(i, edge.size() ) {
		
		pair<int, int> sd = edge[i].second;
		if ( ! uf.issame( sd.first, sd.second ) ) {
			cnt ++;
			ret += edge[i].first;
		}
		
		if ( cnt == N-1 ) { break; }
	}
	
	cout << ret << endl;
}

C++: #include<iostream>
using namespace std;
#define S 100
#define INF 2001
#define WHITE 0;
#define GRAY 1;
#define BLACK 2
int n,a[S][S];

int prim(){
    int u,mina;
    int d[S],p[S],cnd[S];

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

    d[0]=0;
    while(1){
        mina=INF;
        u=-1;
        for(int i=0;i<n;i++){
            if(mina>d[i]&&cnd[i]!=BLACK){
                u=i;
                mina=d[i];
            }
        }
        if(u==-1)break;
        cnd[u]=BLACK;
        for(int v=0;v<n;v++){
            if(cnd[v]!=BLACK&&a[u][v]!=INF){
                if(d[v]>a[u][v]){
                    d[v]=a[u][v];
                    p[v]=u;
                    cnd[v]=GRAY;
                }
            }
        }
    }
            int sum=0;
        for(int i=0;i<n;i++){
            if(p[i]!=-1)sum+=a[i][p[i]];
        }
    return sum;
}

int main(){
    cin>>n;
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            int e;
            cin>>e;
            if(e==-1)a[i][j]=INF;
            else a[i][j]=e;
        }
    }
    cout<<prim()<<endl;
    return 0;

}

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

#if 0
所用時間: 1h

感想: 思ったより実装すべきコード量が多かった。DisjointSetは今後も使う機会が多そうなので、クラス化してみた。
Pの配列を動的に確保するよう工夫した、良い復習になった。

#endif

int N, buff, M = 0, SUM = 0, A[101][101], cost[5000], dpt[5000], dst[5000];//A[Y][X]
vector<pair<int, int> > edges;

class disjointSet {
private:
    int* P;
    
public:
    disjointSet(){
        P = new int[100000];
    }
    disjointSet(int N){     
        P = new int[N];
    }
    ~disjointSet(){
        delete[] P;
    }

    void init(int N){ //木構造の初期化
        for(int i = 0; i <= N; ++i){
            P[i] = i;
        }
    }

    int root(int a){ //根の参照
        if(P[a] == a) return a;
        return (P[a] = root(P[a]));
    }

    bool is_same_set(int a, int b){ //同じ根か?
        return root(a) == root(b);
    }

    void unite(int a, int b){
        P[root(a)] = root(b); //aのrootをbの子にする
    }
};


void preprocess(){//辺の前処理
  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];
        dpt[M] = r;
        dst[M] = c;
        M++;
      }
    }
  }

  //for(int i = 0; i < M; ++i) cout << cost[i] << endl;

  for(int i = 0; i < M; ++i){
    edges.push_back(make_pair(cost[i], i));
  }
  for(int i = 0; i < M; ++i) cout << edges[i].first << " " << edges[i].second << endl;

  sort(edges.begin(), edges.end());

  for(int i = 0; i < M; ++i) cout << edges[i].first << " " << edges[i].second << endl;

}

void solve(){
  disjointSet Union(100000);

  int ans = 0;
  preprocess();

  Union.init(N);

  for(int i = 0; i < M; ++i){
    int index = edges[i].second;
    if(Union.is_same_set(dpt[index], dst[index])) continue;
    else {
      Union.unite(dpt[index], dst[index]);
      cout << "unite " << dpt[index] << " and " << dst[index] << endl;
      ans += cost[index];
    }
  }
  cout << ans << endl;
}

int main(){

cin >> N;

for(int i = 0; i < N; ++i){
  for(int j = 0; j < N; ++j){
    cin >> buff;
    A[i][j] = buff;
    }
  }

  //行列の表示
  // cout << "Matrix:" << endl;
  // for(int row=0; row < N; ++row){
  //   for(int col = 0; col < N; ++col){
  //     cout << A[row][col];
  //   }
  //   cout << endl;
  // }

  solve();
}

C++: #include<stdio.h>

int field[110][110];
int selected[110] = { 0 };
int v;

int prim()
{
	int no_edge = 0;
	int weight = 0;
	selected[0] = 1;
	int x, y;

	while (no_edge < v - 1)
	{
		int min = 0x7fffffff;
		for (int i = 0; i < v; i++)
		{
			if (selected[i])
				for (int j = 0; j < v; j++)
				{
					if (!selected[j] && field[i][j] != -1)
						if (min > field[i][j])
						{
							min = field[i][j];
							x = i;
							y = j;
						}
				}
		}
		weight += field[x][y];
		no_edge++;
		selected[y] = 1;
	}
	return weight;
}

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

C++: #include <iostream>

using namespace std;

int n;

int w[100][100];

int WHITE = 0;
int GRAY = 1;
int BLACK = 2;

int INFTY = 10000;

int color[100];
int d[100];
int p[100];

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

  d[0] = 0;
  p[0] = -1;
  int u;

  int sum = 0;

  while(1) {
    int mincost = INFTY;
    for (int i = 0; i < n; i++) {
      if (color[i] != BLACK && d[i] < mincost) {
        mincost = d[i];
        u = i;
      }
    }

    if (mincost == INFTY) {
      break;
    }

    sum += mincost;

    color[u] = BLACK;
    for (int v = 0; v < n; v++) {
      if (color[v] != BLACK && w[u][v] != INFTY) {
        if(w[u][v] < d[v]) {
          d[v] = w[u][v];
          p[v] = u;
          color[v] = GRAY;
        }
      }
    }
  }

  return sum;
}

int main() {
  cin >> n;

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

  cout << prim() << endl;
}