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++: #pragma region kyomukyomupurin

/**
 *    author  : 𝒌𝒚𝒐𝒎𝒖𝒌𝒚𝒐𝒎𝒖𝒑𝒖𝒓𝒊𝒏
 *    created : 2020-04-24 11:40:03
 **/

#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <chrono>
#include <cmath>
#include <complex>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>

using namespace std;

using int64 = long long;

template <class T>
using binary_heap = std::priority_queue<T, std::vector<T>, std::greater<T>>;

template <class T, class U>
std::ostream& operator<<(std::ostream& os, const std::pair<T, U>& p) {
  return os << '(' << p.first << ", " << p.second << ')';
}

template <class T>
std::ostream& operator<<(std::ostream& os, const std::vector<T>& vec) {
  int flag = 0;
  for (auto e : vec) os << (flag++ ? ", " : "{") << e;
  return os << '}';
}

template <class T>
std::ostream& operator<<(std::ostream& os, const std::set<T>& st) {
  int flag = 0;
  for (auto e : st) os << (flag++ ? ", " : "{") << e;
  return os << '}';
}

template <class T, class U>
std::ostream& operator<<(std::ostream& os, const std::map<T, U>& mp) {
  int flag = 0;
  for (auto e : mp) os << (flag++ ? ", " : "{") << e;
  return os << '}';
}

template <class T>
std::istream& operator>>(std::istream& is, std::vector<T>& vec) {
  for (T& e : vec) is >> e;
  return is;
}

#define all(_) begin(_), end(_)
#define rall(_) rbegin(_), rend(_)

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]: ", debug_out(__VA_ARGS__)
#else
#define debug(...)
#endif

void debug_out() { std::cerr << '\n'; }

template <class Head, class... Tail>
void debug_out(Head&& head, Tail&&... tail) {
  std::cerr << head;
  if (sizeof...(Tail) != 0) std::cerr << ", ";
  debug_out(std::forward<Tail>(tail)...);
}

#pragma endregion kyomukyomupurin


class UnionFind {
 public:
  UnionFind(int n) : n_(n) { initialize(); }

  int root(int x) {
    assert(0 <= x && x < n_);
    if (parent_[x] == -1)
      return x;
    else
      return parent_[x] = root(parent_[x]);
  }

  bool same(int x, int y) {
    assert(0 <= x && x < n_ && 0 <= y && y < n_);
    return root(x) == root(y);
  }

  void unite(int x, int y) {
    assert(0 <= x && x < n_ && 0 <= y && y < n_);
    x = root(x);
    y = root(y);
    if (x == y) return;
    if (rank_[x] < rank_[y]) std::swap(x, y);
    if (rank_[x] == rank_[y]) ++rank_[x];
    parent_[y] = x;
    return;
  }

 private:
  const int n_;
  std::vector<int> parent_;
  std::vector<int> rank_;

  void initialize() {
    parent_.assign(n_, -1);
    rank_.assign(n_, 0);
  }
};

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);

  int n; cin >> n;
  vector<vector<int>> a(n, vector<int>(n)); cin >> a;

  vector<pair<int, pair<int, int>>> v;

  for (int i = 0; i < n; ++i) {
    for (int j = i; j < n; ++j) {
      if (a[i][j] == -1) continue;
      v.emplace_back(a[i][j], make_pair(i, j));
    }
  }

  sort(all(v));
  debug(v);

  UnionFind uf(n);

  int ans = 0;

  for (auto e : v) {
    if (uf.same(e.second.first, e.second.second)) continue;
    ans += e.first;
    uf.unite(e.second.first, e.second.second);
  }

  cout << ans << endl;

  return 0;
}

C++: #include<iostream>
#include<vector>
#include<algorithm>

#define MAX_NODE 10000

using namespace std;

typedef struct{
	int from,to,weight;
} Edge;

typedef struct{
	int parent,rank;
} Node;

Node node[MAX_NODE];
vector<Edge> edge;

void init(int n){
	for(int i=0;i<n;i++){
		node[i].parent=i;
		node[i].rank=0;
	}
}

int find(int n){
	return node[n].parent==n?n:node[n].parent=find(node[n].parent);
}

void unite(int x,int y){
	int xr=find(x);
	int yr=find(y);
	if(node[xr].rank>node[yr].rank){
		node[yr].parent=node[xr].parent;
	}else if(xr!=yr){
		node[xr].parent=node[yr].parent;
		if(node[xr].rank==node[yr].rank){
			node[yr].rank++;
		}
	}
}

bool operator<(const Edge& left,const Edge& right){
	return (left.weight<right.weight?1:0);
}

int main(){
	int n,tmp,w=0;
	Edge e;
	cin>>n;
	for(int i=0;i<n;i++){
		for(int j=0;j<n;j++){
			cin>>tmp;
			if(tmp!=-1){
				e.from=i;
				e.to=j;
				e.weight=tmp;
				edge.push_back(e);
			}
		}
	}
	sort(edge.begin(),edge.end());
	init(n);
	for(int i=0;i<edge.size();i++){
		if(find(edge[i].from)==find(edge[i].to)){
			continue;
		}
		w+=edge[i].weight;
		unite(edge[i].from,edge[i].to);
	}
	cout<<w<<endl;
	return 0;
}