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++: #include<iostream>

using namespace std;

static const int MAX = 100;
static const int INFTY = (1 << 21);
static const int WHITE = 0;
static const int GREY = 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; i < n; i++) {
      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() {
  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;

  return 0;
}

C++: #include <iostream>
#include <stack>
using namespace std;
static const int MAX_V = 100;
static const int INF = 10000000;
int cost[MAX_V][MAX_V];
bool wasUsed[MAX_V];
int mincost[MAX_V];
//n???????????°
int n;

int prim(){
    for(int i=0;i<n;i++){
        wasUsed[i] = false;
        mincost[i] = INF;
    }
    mincost[0] = 0;
    int res = 0;
    while(true){
        int v = -1;
        for(int u =0;u<n;u++){
            if(!wasUsed[u] && (v==-1||mincost[u] < mincost[v]))
               v = u;
        }
        if(v==-1) break;
        wasUsed[v] = true;
        res += mincost[v];
        
        for(int u=0;u<n;u++)
            mincost[u] = min(mincost[u],cost[v][u]);
    }
    return res;
}

int main(int argc, const char * argv[]) {
    cin >> n;
    
    for(int i=0;i<n;i++)
        for(int j=0;j<n;j++){
            int a;
            cin >> a;
            if(a == -1) a = INF;
            cost[i][j] = a;
        }
    
    cout << prim() << endl;
    return 0;
}

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

static const int MAX = 100;
static const int INF = (1<<21);
static const int WHI = 0;
static const int GRA = 1;
static const int BLA = 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] = INF;
    p[i] = -1;
    color[i] = WHI;
  }
  d[0] = 0;
  
  while (1) {
    minv = INF;
    u = -1;
    for ( int i = 0; i < n ; ++i) {
      if ( minv > d[i] && color[i] != BLA ) {
	u = i;
	minv = d[i];
      }
    }
    if ( u == -1 ) {
      break; 
    }
    color[u] = BLA;
    
    for ( int v = 0; v < n ; ++v) {
      if ( color[v] != BLA && m[u][v] != INF) {
	if ( m[u][v] < d[v] ) {
	  d[v] = m[u][v];
	  p[v] = u;
	  color[v] = GRA;
	}
      }
    }
  }
  int sum = 0;
  for ( int i = 0; i < n ; ++i) {
    if ( p[i] != -1 ) {
      sum += m[i][p[i]];
    }
  }
  return sum;
}

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

  cout << prim() << endl;

  return (0);
}

C++: #include<iostream>
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];

// プリムのアルゴリズム（厳密解法)
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;
       // 最適点(コスト最小点)を探す
        for(int i=0;i<n;i++){
            if(minv > d[i] && color[i] != BLACK){
                u = i;
                minv = d[i];
            }
        }
        if(u == -1) break; // uに対しての辺がなかった場合
        color[u] = BLACK;
        
        // 次点の捜索
        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(){
    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;
    return 0;
}