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 <utility>
#include <algorithm>
using namespace std;

int m,n,x,ans;
int P[100+10],edge1[10000+10],edge2[10000+10];


int root(int a){
	if (P[a] == a) return a;
	return (P[a] = root(P[a]));
}

bool same(int a, int b){
	return root(a)==root(b);
}
	
void unite(int a, int b){
	P[root(a)] = root(b);
}

int main() {
	pair<int,int> edges[10000+10];
	cin >> n;
	
	for (int i=1; i<=n; i++){
		P[i] = i;
	}
	
	m = 0;
	for (int i=1; i<=n; i++){
		for (int j=1; j<=n; j++){
			cin >> x;
			if (x>-1){
				edge1[m] = i;
				edge2[m] = j;
				edges[m] = make_pair(x, m);
				m+=1;
			}
		}
	}
	
	sort(edges, edges+m);
	n=1;
	for (int i=0; i<m; i++){
		if (n>=m) break;
		if (!same(edge1[edges[i].second], edge2[edges[i].second])){
			unite(edge1[edges[i].second], edge2[edges[i].second]);
			ans += edges[i].first;
			n+=1;
		}
	}
	
	cout << ans << endl;
	
}

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

using namespace std;
using LL = long long;
using P = pair<int, int>;
using Tapris = tuple<int, int, int>;

#define REP(i, n) for(LL i = 0; i < n; ++i)
#define FOR(i, a, n) for(LL i = a; i < n; ++i)
#define pb(a) push_back(a)
#define all(x) (x).begin(),(x).end()

const int INF = (int)1e9;
const LL INFL = (LL)1e15;
const int MOD = 1e9 + 7;

int dy[]={0, 0, 1, -1, 0};
int dx[]={1, -1, 0, 0, 0};

//#define int long long

class UnionFind {
    private:
        vector<int> parent;  // 親
        vector<int> rank;    // 木の深さ
    public:
        UnionFind(int n);          // n要素で初期化
        int Find(int x);           // 木の根を返す
        void Unite(int x, int y);  // xとyの属する集合を併合
        bool Same(int x, int y);   // xとyが同じ集合に属するか否か
};

UnionFind::UnionFind(int n) : parent(vector<int>(n)), rank(vector<int>(n)) {
    for (int i = 0; i < n; i++) {
        parent[i] = i;
        rank[i] = 0;
    }
}

int UnionFind::Find(int x) {
    if (parent[x] == x) {
        return x;
    } else {
        return parent[x] = Find(parent[x]);
    }
}

void UnionFind::Unite(int x, int y) {
    x = Find(x);
    y = Find(y);
    if (x == y) return;

    if (rank[x] < rank[y]) {
        parent[x] = y;
    } else {
        parent[y] = x;
        if (rank[x] == rank[y]) rank[x]++;
    }
}

bool UnionFind::Same(int x, int y) { return Find(x) == Find(y); }

template <typename T>
struct Edge {
    int src, to;
    T cost;
    Edge(int to, T cost) : src(-1), to(to), cost(cost) {}
    Edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}
};

template<typename T>
using Edges = vector<Edge<T>>;
template<typename T>
using WeightedGraph = vector<Edges<T>>;
using UnWeightedGraph = vector<vector<int>>;
template<typename T>
using Matrix = vector<vector<T>>;


template <typename T>
class Kruskal {
    private:
        int V; // 頂点数
        Edges<T> edges;
        WeightedGraph<T> minimumSpanningTree;
    public:
        Kruskal(int n);
        void AddEdge(int from, int to, T cost);
        T Run();
        WeightedGraph<T> GetMinimumSpanningTree();
};

template <typename T>
Kruskal<T>::Kruskal(int n) : V(n), minimumSpanningTree(n) {}

template <typename T>
void Kruskal<T>::AddEdge(int from, int to, T cost) {
    edges.push_back(Edge<T>(from, to, cost));
}

template <typename T>
T Kruskal<T>::Run() {
    sort(edges.begin(), edges.end(), [](const Edge<T> &a, const Edge<T> &b){
        return a.cost < b.cost;
    });
    UnionFind tree(V);
    T total = 0;
    
    for(auto &e : edges){
        if(!tree.Same(e.src, e.to)){
            tree.Unite(e.src, e.to);
            total += e.cost;
            minimumSpanningTree[e.src].push_back(Edge<T>(e.to, e.cost));
            minimumSpanningTree[e.to].push_back(Edge<T>(e.src, e.cost));
        }
    }

    return total;
}

template <typename T>
WeightedGraph<T> Kruskal<T>::GetMinimumSpanningTree(){
    return minimumSpanningTree;
}

/*************** using variables ***************/

/**********************************************/

signed main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int n;
    cin >> n;
    Kruskal<int> kruskal(n);
    REP(i, n){
        REP(j, n){
            int x;
            cin >> x;
            if(x != -1) kruskal.AddEdge(i, j, x);
        }
    }
    int total = kruskal.Run();
    cout << total << endl;
}