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++: // ALDS1_12_A.cpp
// Graph II - Minimum Spanning Tree
#include <iostream>
using namespace std;

const int maxN = 100;
const int INF = 2001;
int w[maxN][maxN];
int color[maxN];
int n;
static const int WHITE = 0;
static const int BLACK = 1;

int prim(int x)
{
    int res = 0;
    int cnt = 1;
    color[x] = BLACK;
    while (cnt++ != n) {
        int minCost = INF;
        int pos = 0; // ??°?????°?????????
        for (int i = 0; i < n; i++) {
            if (color[i] != BLACK) continue;
            for (int j = 0; j < n; j++) {
                if (color[j] != WHITE) continue;
                if (w[i][j] < minCost) {
                    pos = j;
                    minCost = w[i][j];
                }
            }
        }
        res += minCost;
        color[pos] = BLACK;
    }
    return res;
}

int main()
{
    cin >> n;
    int num;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            cin >> num;
            if (num == -1) w[i][j] = INF;
            else w[i][j] = num;
        }
    }
    cout << prim(0) << endl;
    return 0;
}

C++: #include<iostream>
using namespace std;

class union_find  // union find.
{
	int* U;
public:
	union_find(int n){ U = new int [n + 1]; for(int i = 1; i <= n; i++) U[i] = i; }
	int root(int a){ if(U[a] == a){ return a; } U[a] = root(U[a]); return U[a]; }
	bool same(int a, int b){ return root(a) == root(b); }
	void join(int a, int b){ int x = root(a), y = root(b); U[x] = y; }
	~union_find(){ delete [] U; }
};

struct edge{ int v1, v2; int weight; };  // edge.

edge init(){
	edge e;
	e.v1 = 0; e.v2 = 0; e.weight = -1;
	return e;
}

edge init(int a, int b, int w){
	edge e;
	e.v1 = a; e.v2 = b; e.weight = w;
	return e;
}

void swap_e(edge e1, edge e2)
{
	swap(e1.v1, e2.v1); swap(e1.v2, e2.v2); swap(e1.weight, e2.weight);
}

class edge_heap  // heap.
{
	edge* E;
	int edge_n;
public:
	edge_heap(int n){ E = new edge [n + 1]; edge_n = 0; }
	void min_heap(int i);
	void insert_heap(edge e);   // sorting.
	void insert_nl(edge e);     // no sorting.
	edge delete_min();
	bool isEmpty(){ return edge_n == 0; }
	int total_weight();
	~edge_heap(){ delete [] E; }
};

void edge_heap::min_heap(int i)
{
	int l = 2 * i, r = 2 * i + 1;
	if(l > edge_n && r > edge_n) return;
	int minid = i;
	if(l <= edge_n){ if(E[l].weight < E[minid].weight) minid = l; }
	if(r <= edge_n){ if(E[r].weight < E[minid].weight) minid = r; }
	if(minid == i) return;
	swap(E[minid], E[i]);
	min_heap(minid);
}

void edge_heap::insert_heap(edge e)
{
	edge_n++;
	int w = e.weight;
	int i = edge_n;
	while(i > 1){
		if(w < E[i / 2].weight){
		E[i] = E[i / 2];
		}else{ break; }
		i /= 2;
	};
	E[i] = e;
}
void edge_heap::insert_nl(edge e){ edge_n++; E[edge_n] = e; }

edge edge_heap::delete_min()
{
	edge e = E[1];
	E[1] = E[edge_n]; edge_n--;
	min_heap(1);
	return e;
}

int edge_heap::total_weight()
{
	int tw = 0;
	for(int i = 1; i <= edge_n; i++) tw += E[i].weight;
	return tw;
}

int main()
{
	int n, i, j, w;
	cin >> n;
	edge_heap EH(n * (n + 1) / 2); // All Edge heap.
	edge e;

	for(i = 1; i <= n; i++){
		for(j = 1; j <= n; j++){
			cin >> w;
			if(j <= i || w < 0) continue;
			e = init(i, j, w);
			EH.insert_heap(e);
		}
	}

	edge_heap TH(n);   // Tree heap.

	union_find VG(n);  // vertex group.

	while(!EH.isEmpty()){
		e = EH.delete_min();
		if(!VG.same(e.v1, e.v2)){
			VG.join(e.v1, e.v2);
			TH.insert_nl(e);
		};
	};

	cout << TH.total_weight() << endl;

	return 0;
}

C++: #include<bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i) 
#define PI acos(-1)

using namespace std;
using ll = long long;
using P = pair<int, int>;
using LP = pair<ll, ll>;

struct DisjointSet {
private:
    vector<int> par;
    vector<int> rank;

public:
    DisjointSet(int n) {
        par = vector<int>(n);
        rep(i, n) par[i] = i;
        rank = vector<int>(n);
    }

    int find(int i) {
        if (par[i] == i) return i;
        int p = find(par[i]);
        par[i] = p;
        return p;
    }

    bool joint(int i, int j) {
        int pi = find(i);
        int pj = find(j);
        if (pi == pj) return false;

        if (rank[pi] < rank[pj]) this->par[pi] = pj;
        else if (rank[pj] < rank[pi]) this->par[pj] = pi;
        else {
            this->par[pi] = pj;
            rank[pj]++;
        }
        return true;
    }
};

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int n;
    cin>>n;
    priority_queue<vector<int>, vector<vector<int>>, greater<vector<int>>> q;
    rep(i, n) {
        rep(j, n) {
            int w;
            cin>>w;
            if (w == -1) continue;

            vector<int> v = {w, i, j};
            q.push(v);
        }
    }
    DisjointSet ds(n);
    ll ans = 0;
    while (!q.empty()) {
        vector<int> v = q.top(); q.pop();
        int w = v[0];
        int v1 = v[1];
        int v2 = v[2];
        if (ds.joint(v1, v2)) ans += w;
    }
    cout<<ans<<endl;
}