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>
using namespace std;


struct edge
{
   int cost;
   int src;
   int dst;
};

edge e[5000];
int A[100][100];
int M=0;
int P[100];

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);
}

void checkEdges(int n)
{
    for(int r=0;r<n;r++)
        for(int c=r+1;c<n;r++)
    {
        if(A[r][c]>-1)
        {
            e[M].cost=A[r][c];
            e[M].src=r;
            e[M].dst=c;
            M+=1;
        }
    }
}

bool cmp(edge e1,edge e2)
{
    return e1.cost<=e2.cost;
}
int main()
{
    int n,count=0;
    cin>>n;
    init(n);
    for(int i=0;i<n;i++)
    {
        for(int j=0;j<n;j++){
                cin>>A[i][j];
        }
    }
    checkEdges(n);
    sort(e,e+M,cmp);
    for (int i=0; i<M; ++i)
    {
        if(is_same_set(e[i].src,e[i].dst))
        {
            continue;
        }
        unite(e[i].dst,e[i].src);
        count+=e[i].cost;
    }
    cout<<count<<endl;
    //system("pause");
    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
#define int long long
signed 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[i][j];
  int ans=0;
  bool used[n];
  memset(used,0,sizeof(used));
  used[0]=1;
  int cost[n];
  for(int i=0;i<n;i++) cost[i]=a[0][i];
  while(1){
    int v=-1;
    for(int i=0;i<n;i++){
      if(used[i]||cost[i]<0) continue;
      //cout<<i<<":"<<cost[i]<<endl;
      if(v<0||cost[i]<cost[v]) v=i;
    }
    if(v<0) break;
    used[v]=1;
    ans+=cost[v];
    //cout<<v<<endl;
    for(int i=0;i<n;i++){
      if(a[v][i]<0) continue;
      if(cost[i]<0||a[v][i]<cost[i]) cost[i]=a[v][i];
    }
  }
  cout<<ans<<endl;
  return 0;
}

C++: #include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define repr(i, n) for (int i = (n); i >= 0; --i)
#define FOR(i, m, n) for (int i = (m); i < (n); ++i)
#define FORR(i, m, n) for (int i = (m); i >= (n); --i)
using namespace std;
typedef long long ll;
const ll mod = 1000000007;
const ll mod2 = 998244353;
const int INF = 1000000005;
const int MAX = 105;

class edge {
  public:
  int t, c;

  edge(int t = -1, int c = INF): t(t), c(c) {}

  bool operator < (const edge &e) const {
    return c > e.c;
  }
};

vector<edge> to[MAX];
bool seen[MAX];
int n;

int prim() {
  seen[0] = true;
  priority_queue<edge> pq;
  int sum = 0, u = 0;

  rep(l, n-1) {
    for (edge e: to[u]) {
      if (seen[e.t]) continue;
      pq.push(e);
    }

    edge d;
    while (1) {
      d = pq.top();
      pq.pop();
      if (!seen[d.t]) break;
    }

    seen[d.t] = true;
    sum += d.c;
    u = d.t;
  }

  return sum;
}

int main() {
  cin >> n;
  edge e;
  rep(i, n) {
    rep(j, n) {
      cin >> e.c;
      if (e.c == -1) continue;
      e.t = j;
      to[i].push_back(e);
    }
  }

  rep(i, n) seen[i] = false;

  cout << prim() << endl;

  
  return 0;
}

C++: #include<iostream>
#include<cstring>
#include<algorithm>
using namespace std;
 
int x[10000][10000];
int MIN[10000];
int MINX = 0;
 
int main() {
    memset(MIN, 127, sizeof(MIN));
    int n, a;
    cin >> n;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            cin >> a;
            x[i][j] = a;
        }
    }
    MIN[0] = 0;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            for (int k = 0; k < n; k++) {
                if (x[j][k] != -1) {
                    MIN[k] = min(MIN[k], x[j][k]);
                }
            }
        }
    }
    for (int i = 0; i < n; i++) {
        MINX = max(MINX, MIN[i]);
    }
    cout << MINX << endl;
    return 0;
}

C++: #include <iostream>
#include <queue>

using namespace std;




struct link { //link connecting x and y vertices
  int x;
  int y;
  int weight;
  bool operator<(const link& other) const
  {
    return weight > other.weight; // for min priority queue, trick the operator
  }
};

priority_queue<link> links;
int roots[100];

int root(int x) {
  if (x == roots[x]) {
    return x;
  } else {
    //roots[x] = root(roots[x]); //attach all to root
    return root(roots[x]);
  }
}

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

void unite(int x, int y) {
  roots[root(x)] = root(y);
}


  
int main() {
  int min_cost = 0;
  int n;
  cin >> n;


  for (int i = 0; i < n; i++) {
    roots[i] = i; // initialize roots
    for (int j = 0; j < n; j++) {
      
      int weight;
      cin >> weight;
      if (j <= i ) continue;
      if (weight != -1) {
	link new_link = {i, j, weight};
	links.push(new_link);
	//cout << "link added has i j weight: " << i << " " << j << " " << weight << endl;
      }
    }
  }

  while (!links.empty()) {
    link cur_link = links.top();
    //cout << "cur link weight: " << cur_link.weight << " x: " << cur_link.x << " y: " << cur_link.y << endl;
    if (!same(cur_link.x, cur_link.y)) {
      min_cost += cur_link.weight;
      unite(cur_link.x, cur_link.y);
    }
    links.pop();
  }

  cout << min_cost << endl;
      
    
}

C++: #include<iostream>
using namespace std;
#define MAX 100
#define White 1
#define Black 2
#define Red 3
#define Infinit 2001
struct Vertex {
	int weight,color,parent;
};
int M[MAX][MAX];
int n;
Vertex v[MAX];
void search() {
	int weight[MAX],pointer,min;
	for (int i = 0; i < n; i++) {
		if (i == 0) {
			v[i].weight = 0;
		}
		else {
			v[i].weight = Infinit;
		}
		v[i].color = White;
		v[i].parent = -1;
	}
	while (true) {
		min = Infinit;
		pointer = -1;
		for (int i = 0; i < n; i++) {
			if (v[i].color != Black && min>v[i].weight) {
					min = v[i].weight;
					pointer = i;
			}
		}
		if (min==Infinit) break;
		v[pointer].color = Black;
		for (int k = 0; k < n; k++) {
			if (M[pointer][k] != -1 && v[k].color!=Black) {
				if (M[pointer][k]<v[k].weight) {
					v[k].weight = M[pointer][k];
					v[k].parent = pointer;
					v[k].color = Red;
				}
			}
		}

	}
	int sum = 0;
	for (int i = 0; i < n; i++) {
		sum += v[i].weight;
	}
	cout << sum << endl;


}
int main() {
	int id;
	cin >> n;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			cin >>id;
			M[i][j]=id;
		}
	}
	search();
}