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>
#include<vector>
#include<algorithm>

using namespace std;

struct edge{
	int u;	//?????????
	int v;	//??°??????
	int cost;	//??????
	//???????????????????????§?????§?°???¢????????????
	bool operator<(const edge& another) const {
			return cost < another.cost;
	}
};

const int MAX_N = 100;
const int MAX_M = 10000;

edge es[MAX_M];
int par[MAX_N];
int rank[MAX_N];

void initial(int n){		//?????????
	for(int i=0;i<n;i++){
		par[i] = i;
		rank[i] = 0;
	}
}

int find(int x){		//??¨??????????±???????
	if(par[x] == x){
		return x;
	}else{
		return par[x] = find(par[x],par);
	}
}

void unite(int x,int y){	//x???????????¨??¨y???????????¨?????????
	x = find(x,par);	//x???????????¨??????????????????x??¨??????
	y = find(y,par);	//y???????????¨??????????????????y??¨??????
	if(x == y) return;
	if(rank[x] < rank[y]){
		par[x] = y;
	}else{
		par[y] = x;
		if(rank[x] == rank[y])
			rank[x]++;
	}
}

bool same(int x,int y){	//x,y???????????¨???????????§?????????
	return find(x,par) == find(y,par);
}

bool comp(const edge& e1,const edge& e2){
	return e1.cost < e2.cost;
}

int kruskal(int n,int m){
	initial(n,par,rank);
	int res = 0;
	for(int i=0;i<m;i++){
		edge e = es[i];
		if(!same(e.u,e.v,par)){
			unite(e.u,e.v,par,rank);
			res += e.cost;
		}
	}
	return res;
}

int main(){
	int n,m=0,k,l,x;
	cin >> n;	//????????°?????\???
	int cnt=0;
	for(int i=0;i<n;i++){
		for(int j=0;j<n;j++){
			cin >> x;
			if(x != -1){
				edge e;
				e.from = i; e.to = j; e.cost = x;
				es[m] = e;
				m++;
			}
		}
	}
	sort(es,es+m);
	cout << kruskal(n,m) << "\n";
}

C++: #include <bits/stdc++.h>
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];

int prim() {
  int u, minv;
  // 重み、親、色
  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 (color[i] != BLACK && d[i] < minv) {
        minv = 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] != 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++) {
    sum += d[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<stdio.h>
#include<algorithm>
using namespace std;
int maxn=1<<21;
int n;
int a[105][105];
int color[105];
int d[105];
int p[105];
int f()
{
    int u;int minx;
    for(int i=0;i<105;i++)
    {
        d[i]=maxn;
        p[i]=-1;
    }
    d[0]=0;
    while(1)
    {
        minx=maxn;
        u=-1;
        for(int i=0;i<n;i++)
        {
            if(minx>d[i]&&color[i]==0)
            {
                u=i;
                minx=d[i];
            }
        }
        if(u==-1) break;
        color[u]=1;
        for(int i=0;i<n;i++)
        {
            if(color[i]==0&&a[u][i]!=maxn)
            {
                if(d[i]>a[u][i])
                {
                    d[i]=a[u][i];
                    p[i]=u;
                    color[i]=0;
                }
            }
        }
    }
    int sum=0;
    for(int i=0;i<n;i++)
    {
        if(p[i]!=-1) sum+=a[i][p[i]];
    }
    return sum;
}
int main()
{
    scanf("%d",&n);
    for(int i=0;i<n;i++)
    {
        for(int j=0;j<n;j++)
        {
            int x;
            scanf("%d",&x);
            a[i][j]=(x==-1)?maxn:x;
        }
    }
    printf("%d\n",f());
    return 0;
}