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 _GLIBCXX_DEBUG
typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<pair<int, int>, int> PP;
const ll INF = 1LL << 60;
const ll MOD = 1000000007;
const int MAX = 100000;

int GCD(int a, int b) { return b ? GCD(b, a % b) : a; }
//------------------------------------------------------------------------------------------------------------------------------

vector<int> par(MAX);
vector<int> rk(MAX);
vector<int> cnt(MAX);

void init(int n) {
    for (int i = 0; i < n; i++) {
        par[i] = i;
        rk[i] = 0;
        cnt[i] = 1;
    }
}

int find(int x) {
    if (par[x] == x) {
        return x;
    } else {
        return par[x] = find(par[x]);
    }
}

void unite(int x, int y) {
    x = find(x);
    y = find(y);
    if (x == y)
        return;

    if (rk[x] < rk[y]) {
        par[x] = y;
        cnt[x] += cnt[y];
    } else {
        par[y] = x;
        cnt[y] += cnt[x];
        if (rk[x] == rk[y])
            rk[x]++;
    }
}

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

struct edge {
    int u, v, cost;
};

bool comp(const edge &e1, const edge &e2) { return e1.cost < e2.cost; }
vector<edge> es;
int kruskal(int V, int E) {
    sort(es.begin(), es.end(), comp);
    init(V);
    int res = 0;
    for (int i = 0; i < E; i++) {
        edge e = es[i];
        if (!same(e.u, e.v)) {
            unite(e.u, e.v);
            res += e.cost;
        }
    }
    return res;
}

int main(void) {
    int N;
    cin >> N;
    int E = 0;
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < N; j++) {
            int c;
            cin >> c;
            if (c >= 0) {
                es.push_back({i, j, c});
                E++;
            }
        }
    }
    cout << kruskal(N, E) << endl;
}

C++: #include<iostream>

using namespace std;

static const int MAX = 100;
static const int NIL = (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] = INFY;
    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;
}