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 <bits/stdc++.h>
using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP0(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)
#define REP1(i,n) for(int i=1, i##_len=(n); i<=i##_len; ++i)
#define RREP0(i,n) for(int i=n-1; i>=0; --i)
#define RREP1(i,n) for(int i=n; i>=1; --i)

#define SORT(c) sort((c).begin(),(c).end())
#define whole(f,x,...) ([&](decltype((x)) whole) { return (f)(begin(whole), end(whole), ## __VA_ARGS__); })(x)
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define CLR(a) memset((a), 0 ,sizeof(a))

inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}
int gcd(int a,int b){return b?gcd(b,a%b):a;}
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};

typedef long long LL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;

const int INTINF = 2147483647;
const LL LLINF = 9223372036854775807;
const double EPS = 1e-10;
const double PI  = acos(-1.0);
const int N = 100;
int color[N]; // i's visit status
int M[N][N];
int d[N];  // minimum weight
int p[N]; // i's parent vertex in MST
int n;

#define WHITE 0
#define GRAY 1
#define BLACK 2

struct init{
    init(){
        cin.tie(0); ios::sync_with_stdio(0); cout<<fixed<<setprecision(20);
    };
} init;


void prim(){
  int u, minv;

  REP0(i,n){
    color[i] = WHITE;
    d[i] = INTINF;
    p[i] = -1;
  }

  d[0] = 0;

  while (1) {
    minv = INTINF;
    u = -1;
    REP0(i,n){
      if( minv > d[i] && color[i] != BLACK ){
        u = i;
        minv = d[i];  // MSTの隣接辺のうち最小重みを選ぶ
      }
    }
    if ( u==-1 ) break; // 最小重みがなく終了
    color[u] = BLACK; // uをMSTに追加
    REP0(v,n){ // 新たなMSTで, 隣接する頂点の重みと親を更新
      if ( color[v] != BLACK && M[u][v] != INTINF  && d[v] > M[u][v] ) {
        d[v] = M[u][v];
        p[v] = u;
        color[v] = GRAY;
      }
    }
  }

 int sum = 0;
 REP0(i,n){ // MST内で親を持つ頂点について
   if( p[i] != -1 ) sum += M[i][p[i]];
 }
 std::cout << sum << '\n';
}


void input(){
  std::cin >> n;
  int w;
  REP0(i,n) REP0(j,n) {
    std::cin >> w;
    M[i][j] = (w==-1) ? INTINF : w;
  }
}


int main(int argc, char const *argv[]) {
  input();
  prim();
  return 0;
}

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;
}