Go: package main

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

const BIG_COST = 3000

const (
	ST_NOT_YET = iota
	ST_JOINED
)

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)

	nNodes := scanInt(sc)
	costs := make([][]int, nNodes)

	for r := 0; r < nNodes; r++ {
		costs[r] = make([]int, nNodes)
		for c := 0; c < nNodes; c++ {
			costs[r][c] = scanInt(sc)
		}
	}

	states := make([]int, nNodes)
	minCosts := make([]int, nNodes)

	for i := 1; i < nNodes; i++ {
		minCosts[i] = BIG_COST
	}

	for {
		var u int
		minCost := BIG_COST

		for i := 0; i < nNodes; i++ {
			if states[i] != ST_JOINED && minCosts[i] < minCost {
				minCost = minCosts[i]
				u = i
			}
		}

		// 全てのノードが MST に JOIN できたので終了。
		if minCost == BIG_COST {
			break
		}

		states[u] = ST_JOINED

		for v := 0; v < nNodes; v++ {
			cost := costs[u][v]

			// MST内の u と MST外の v との辺の重みをそれぞれ minCosts[v] として
			// 保存しておく。このうち辺の重みが最小となる v が次のループで MST に JOIN する。
			if states[v] != ST_JOINED && cost != -1 && cost < minCosts[v] {
				minCosts[v] = cost
			}
		}
	}

	sum := 0
	for _, cost := range minCosts {
		sum += cost
	}
	fmt.Println(sum)
}

func scanInt(sc *bufio.Scanner) int {
	sc.Scan()
	v, _ := strconv.Atoi(sc.Text())
	return v
}

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

typedef long long ll;

#define MAX_V 100
#define INF int(1e9)

// g : ??°??????, V : g???????????°, s : ????????????????????????
int Prim (int g[MAX_V + 1][MAX_V + 1], int V, int s) {
    ll ret = 0, min_, nxtv, v = s, size_;
    // bool used[V] = {false}; // c++11??????
    bool used[V]; memset(used, 0, sizeof(used)); // c++11??\???
    vector<int> usedv;
    usedv.push_back(s);
    used[s] = true;

    while (usedv.size() < V) {
        min_ = INF;
        size_ = usedv.size();

        // ?????¨???????????????????????°?????????????????\?¶????????????????????°????????????¢???
        for (int i = 0; i < size_; ++i) {
            for (int j = 0; j < V; ++j) {
                if (i == j) continue;
                if (!used[j] && min_ > g[usedv[i]][j]) {
                    min_ = g[usedv[i]][j];
                    v = j;
                }
            }
        }

        ret += min_;
        usedv.push_back(v);
        used[v] = true;
    }

    return ret;
}

int n, g[MAX_V + 1][MAX_V + 1];

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    cin >> n;
    for (int 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;
        }
    }

    cout << Prim(g, n, 0) << endl;


    return 0;
}

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

const int MAX = 105;
const int WHITE = 0;
const int GRAY = 1;
const int BLACK = 2;
const int INFTY = 1 << 21;
int n;
int m[MAX][MAX];
int color[MAX];
int d[MAX];
int p[MAX];

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

  while (true) {
    int u {-1};
    int min_cost = INFTY;
    for (int i = 0; i < n; i++) {
      if (color[i] != BLACK && d[i] < min_cost) {
        min_cost = d[i];
        u = i;
      }
    }
    if (u == -1) break;
    color[u] = BLACK;
    for (int v = 0; v < n; v++) {
      if (color[v] != BLACK && m[u][v] < d[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() {
  cin >> 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;
    }
  }
  cout << prim() << endl;
}

C++: #include<iostream>
#include<algorithm>
#define MAXV 101
#define INF 2001
using namespace std;

int n, total = 0, mine, minj;
int mst[MAXV] = {}; //1 origin
int edge[MAXV][MAXV] = {}; //1 origin

void prim(void);
bool doesFinished(void);

int main() {
	int i, j;
	
	cin >> n;
	for (i = 1; i <= n; i++) {
		
		for (j = 1; j <= n; j++) {
			cin >> edge[i][j];
		}
	}

	mst[1] = 1;
	while (!doesFinished()) {
		prim();
	}

	cout << total << endl;

	return 0;
}

void prim(void) {
	int i, j;
	mine = INF;
	for (i = 1; i <= n; i++) {
		if (mst[i] == 0)continue;
		for (j = 1; j <= n; j++) {
			if (edge[i][j] == -1 || mst[j] == 1)continue;
			if (edge[i][j] < mine) {
				mine = edge[i][j];
				minj = j;
			}
		}
	}
	mst[minj] = 1;
	if(mine != INF)total += mine;
}

bool doesFinished(void) {
	for (int i = 1; i <= n; i++) {
		if (mst[i] == 0)return false;
	}
	return true;
}

C++: #include<iostream>
#include<cstdio>
#include<cstring>
#include <cstdlib>  
#include <cmath>   
#include<cctype>
#include<string>
#include<set>
#include <map>
#include<algorithm>
#include <functional>
#include<vector>
#include<climits>
#include<stack>
#include<queue>
#include <deque>
#include <typeinfo>
#include <utility> 

#define rep(i,m,n) for(int i = m;i < n;++i)
using namespace std;
using ll = long long;
using R = double;
const ll inf = 1LL << 50;
const ll MOD = 1e9 + 7;

int infty = (1 << 21);
int cost[100][100];
int mincost[100];
bool used[100];
int V;

int prim() {
	for (int i = 0; i < V; ++i) {
		mincost[i] = infty;
		used[i] = false;
	}
	mincost[0] = 0;
	int res = 0;

	while (1) {
		int v = -1;
		for (int u = 0; u < V; u++) {
			if (!used[u] && (v == -1 || mincost[u] < mincost[v])) {
				v = u;
			}
		}
		if (v == -1)break;
		used[v] = true;
		res += mincost[v];

		for (int u = 0; u < V; u++) {
			mincost[u] = min(mincost[u], cost[v][u]);
		}
	}
	return res;
}


int main() {
	cin >> V;
	rep(i, 0, V)rep(j, 0, V) {
		int e; cin >> e;
		cost[i][j] = (e == -1) ? infty : e;
	}

	cout << prim() << endl;
}

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

struct Edge {
    int from, to, cost;
    Edge(int f, int t, int c) : from(f), to(t), cost(c) { }
    bool operator <(const Edge& e) const {
        return cost < e.cost;
    }
};

struct UnionFind {
    vector<int> data;
    UnionFind(int n) : data(n, -1) { }
    int root(int x) {
        return data[x] < 0 ? x : data[x] = root(data[x]);
    }

    bool same(int x, int y) {
        return root(x) == root(y);
    }

    void merge(int x, int y) {
        x = root(x), y = root(y);
        if(x != y) {
            data[x] += data[y]; data[y] = x;
        }
    }
};

int main() {
    int N; cin >> N;
    vector<Edge> es;
    for(int i = 0; i < N; i++) {
        for(int j = 0; j < N; j++) {
            int c; cin >> c;
            if(c != -1) {
                es.push_back(Edge(i, j, c));
            }
        }
    }
    sort(es.begin(), es.end());
    UnionFind uf(N);
    int ans = 0;
    for(int i = 0; i < (int)es.size(); i++) {
        int from = es[i].from;
        int to = es[i].to;
        int cost = es[i].cost;
        if(uf.same(from, to)) continue;
        uf.merge(from, to);
        ans += cost;
    }
    cout << ans << endl;
}

C++: #include<bits/stdc++.h>
using namespace std;

typedef pair< int , int > P;
struct Edge {
  int to,cost;
  Edge(){}
  Edge(int to,int cost):to(to),cost(cost){};
};
vector< vector< Edge > > Graph;
int V,E;
int prim(){
  int total = 0;
  vector< bool > used(Graph.size(),false);
  priority_queue< P,vector< P >,greater< P > > que;
  que.push(P(0,0));
  while(!que.empty()){
    int cost = que.top().first;
    int now = que.top().second;
    que.pop();
    if(used[now]) continue;
    used[now] = true;
    total += cost;
    for(int i=0;i<Graph[now].size();i++){
      que.push(P(Graph[now][i].cost,Graph[now][i].to));
    }
  }
  return total;
}

int main(){
  cin >> V;
  Graph.resize(V+1);
  /*
  for(int i=0;i<E;i++){
    int s,t,d;
    cin >> s >> t >> d;
    Graph[s].push_back(Edge(t,d));
    Graph[t].push_back(Edge(s,d));
    }*/
  for(int i=0;i<V;i++){
    for(int j=0;j<V;j++){
      int d; cin >> d;
      if(d != -1){
	Graph[i].push_back(Edge(j,d));
	Graph[j].push_back(Edge(i,d));
      }
    }
  }
  cout << prim() << endl;
}