Go: package main

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

// Define disjoint-set forests

type vertexNo int

type vertex struct {
	number vertexNo
	rank int
	parent *vertex
}

func makeSet(number vertexNo) vertex {
	return vertex {
		number: number,
		rank: 0,
		parent: nil,
	}
}

func union(x, y *vertex) *vertex {
	return link(findSet(x), findSet(y))
}

func link(x, y *vertex) *vertex {
	if x.rank > y.rank {
		y.parent = x
		return x
	} else {
		x.parent = y
		if x.rank == y.rank {
			y.rank += 1
		}
		return y
	}
}

func findSet(v *vertex) *vertex {
	if v.parent == nil {
		return v
	}
	v.parent = findSet(v.parent)
	return v.parent
}

// Function to calculate minimum spanning tree

type matrix [][]int

type edge struct {
	one vertexNo
	two vertexNo
	weight int
}

const(
	MAX_VERTICES = 100
)

func newEdgesFromMatrix(matrix matrix) []edge {
	edges := make([]edge, 0, MAX_VERTICES * MAX_VERTICES)

	for i := 0; i < len(matrix); i++ {
		row := matrix[i]
		for j := i; j < len(row); j++ {
			if row[j] >= 0 {
				edge := edge{one: vertexNo(i+1), two: vertexNo(j+1), weight: row[j]}
				edges = append(edges, edge)
			}
		}
	}

	return edges
}

func solve(matrix matrix) int {
	nVertices := len(matrix)

	// initialize vertices
	vertices := make([]vertex, nVertices)
	for i := 0; i < nVertices; i++ {
		vertices[i] = makeSet(vertexNo(i+1))
	}

	// prepare edges sorted in the ascending order of weight
	edges := newEdgesFromMatrix(matrix)
	sort.Slice(edges, func(i, j int) bool {
		return edges[i].weight < edges[j].weight
	})

	unionCount := 0
	weightSum := 0
	for unionCount < nVertices-1 {
		for i := 0; i < len(edges); i++ {
			edge := edges[i]
			v1 := &vertices[edge.one-1]
			v2 := &vertices[edge.two-1]
			rep1 := findSet(v1)
			rep2 := findSet(v2)
			if rep1.number == rep2.number {
				continue
			}
			union(rep1, rep2)
			edges = append(edges[0:i], edges[i+1:]...)
			weightSum += edge.weight
			unionCount++
			break
		}
	}

	return weightSum
}

// main

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

	n, err := nextInt(sc)
	if err != nil {
		panic(err)
	}

	matrix := parseMatrix(n, sc)

	edges := newEdgesFromMatrix(matrix)
	sort.Slice(edges, func(i, j int) bool {
		return edges[i].weight < edges[j].weight
	})

	fmt.Println(solve(matrix))
}

func parseMatrix(n int, sc *bufio.Scanner) [][]int {
	matrix := make([][]int, n)
	for i := 0; i < n; i++ {
		matrix[i] = make([]int, n)
		for j := 0; j < n; j++ {
			x, err := nextInt(sc)
			if err != nil {
				panic(err)
			}
			matrix[i][j] = x
		}
	}
	return matrix
}

func nextText(sc *bufio.Scanner) string {
	sc.Scan()
	return sc.Text()
}

func nextInt(sc *bufio.Scanner) (int, error) {
	return strconv.Atoi(nextText(sc))
}

C++: 
// O(|E||V|)

#include <vector>
#include <cassert>
#include <algorithm>

using namespace std;

class Prim {
#define MAX_V 101

private:
  const int INF = 1e9 + 10;
  struct Edge {
    int to, cost;
    Edge(int to, int cost): to(to), cost(cost) { }
  };

  int V;
  vector<Edge> G[MAX_V];
  int mincost[MAX_V];
  bool used[MAX_V];

public:

  Prim(int V): V(V) { 
    if (V >= MAX_V) {
      assert(false);
    }
  }

  void AddEdge(int from, int to, int cost) {
    G[from].push_back(Edge(to, cost));
  }

  int Solve() {
    for (int i = 0; i < V; i++) {
      mincost[i] = INF;
      used[i]    = false;
    }

    mincost[0] = 0;
    int res = 0;

    while (true) {
      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 (auto &e : G[v]) {
        mincost[e.to] = min(mincost[e.to], e.cost);
      }
    }

    return res;
  }
};

#include <iostream>

int main() {
  int n;
  cin >> n;
  Prim prim(n);

  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      int c; cin >> c;
      if (c != -1) {
        prim.AddEdge(i, j, c);
      }
    }
  }

  int ans = prim.Solve();

  cout << ans << endl;

  return 0;
}

C++: #include<iostream>
using namespace std;
#include<queue>
#include<vector>
#include<string>
#include<string.h>
#include<algorithm>
bool b[2003];

#define REP(i, a) for(int i=0;i<a;i++)

int main()
{
	int min,b,a,m[101][101],n ,e,p[101],d[101],u,v; //u ?????????v????????¨
	bool color[101];
	cin>>n;
	REP(i, n) {
		REP(j, n) {
			cin>>m[i][j];
			if(m[i][j]==-1) m[i][j]=2001;
		}
		color[i]=0;
		d[i]=2001;
		p[i]=2001;
	}
	a=0;
	d[0]=0;
	while(1){
		min=2001;
		u=-1;
		REP(i,n){
			if(min>d[i]&&color[i]==0){
				min=d[i];
				u=i;
			}
		}
		if(u==-1) break;
		color[u]=1;
		REP(i,n){
			if(color[i]==0&&m[u][i]!=2001){
				if(d[i]>m[u][i]){
					d[i]=m[u][i];
					p[i]=u;
				}
			}
		}

	}
	min=0;
	REP(i,n){
		if(p[i]!=2001){
		min=min+m[i][p[i]];
		}
	}
	cout<<min<<endl;
	return 0;
}

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

int n; // number of vertices 
int P[20200], A[20200], B[20200], C[20200];
pair<int, int> D[20200]; // Weights and coordinates 

int root(int a) { // find the root of a
  if (P[a] == a) return a; // a is the root
  return  (P[a] = root(P[a])); // find root of a, make it a's parent
}

bool is_same_set(int a, int b) { // a and b are in the same group?
  return root(a) == root(b);
}

void unite(int a, int b) { // put a and b in same group
  P[root(a)] = root(b);
}


int main() {
  cin >> n; // the number of vertices
  for (int i=0; i<n; ++i) {
    P[i] = i; // Initialize vertices to disjoint sets
    for (int j=0; j<n; ++j) {
      A[(n * i) + j] = i;
      B[(n * i) + j] = j;
      cin >> C[(n * i) + j];
    }
  }
  for (int i=0; i < n*n; i++) {
    D[i].first = C[i];
    D[i].second = i;
  }
  sort(D, D+n*n);
  int sum = 0;

  for (int i=0; i<n*n; ++i) {
    if (D[i].first >= 0) { // Assuming the weight is not -1
      if (!is_same_set(A[D[i].second], B[D[i].second])) {
	unite(A[D[i].second], B[D[i].second]);
	sum += D[i].first;
      }
    }
  }
  cout << sum << endl;
}

C++: #include <iostream>

using namespace std;

#define N_MAX 100
#define W_MAX 2000

int Adj[N_MAX][N_MAX] = {};

int color[N_MAX] = {};

int sum_mst_path = 0;
int n;

int prim(void);

int main(){
    int i, j;

    cin >> n;
    for (i = 0; i < n; i++) {
        for (j = 0; j < n; j++) {
            cin >> Adj[i][j];
        }
    }

    color[0] = 1;
    while ( prim() );

    cout << sum_mst_path << endl;

    return 0;
}

//[?????????
//0 : MST????§???????????????????
//1 : ???????????????????????????
int prim(void) {
    int i, j, min, next_node;

    min = W_MAX + 1;
    next_node = N_MAX + 1;

    //?¢????node???????????????????¢????node??????????????§???????°??????¢???path?????¢?´¢
    for (i = 0; i < n; i++) {
        if (color[i] != 1)
            continue;

        for (j = 0; j < n; j++) {
            if (color[j] != 0)
                continue;

            if (Adj[i][j] != -1 && Adj[i][j] < min) {
                min = Adj[i][j];
                next_node = j;
            }
        }
    }

    if (next_node == N_MAX + 1) //??????????¢??????????
        return 0;

    //next_node????¢????
    color[next_node] = 1;
    sum_mst_path += min;

    return 1;
}