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 <queue>
#include <vector>
#include <iostream>
using namespace std;

// ------ Class ------ //
class Graph2 {
public:
	// ------ Variables ------ //
	static const long long INF = 1000000000000000000;
	int V, E; vector<vector<pair<int, long long> > > G;

	// ------ Constructors ------ //
	Graph2() : V(0), E(0), G(vector<vector<pair<int, long long> > >()) {}
	Graph2(int v) : V(v), E(0), G(vector<vector<pair<int, long long> > >(v)) {}
	Graph2(vector<vector<pair<int, long long> > > g) : V(g.size()), G(g) { for (int i = 0; i < g.size(); i++) E += g[i].size(); }

	// ------ Basic Functions ------ //
	int size() { return V; }
	void add1(int v1, int v2, long long w) { G[v1].push_back(make_pair(v2, w)); E++; }
	void add2(int v1, int v2, long long w) { add1(v1, v2, w); add1(v2, v1, w); }

	// ------ Operators ------ //
	bool operator==(const Graph2& g) const { return G == g.G; }
	bool operator!=(const Graph2& g) const { return G != g.G; }
	vector<pair<int, long long> > operator[](int x) { return G[x]; }

	// ------ Algorithms ------ //
	long long minspan() {
		vector<long long> d(V, INF);
		vector<bool> used(V, false);
		priority_queue<pair<long long, int> > que;
		for (int i = 0; i < V; i++) d[i] = INF;
		d[0] = 0; que.push(make_pair(0, 0));
		while (!que.empty()) {
			pair<long long, int> pa = que.top(); que.pop();
			int u = pa.second; used[u] = true;
			for (int i = 0; i < G[u].size(); i++) {
				int v = G[u][i].first;
				if (d[v] > G[u][i].second && !used[v]) {
					d[v] = G[u][i].second; que.push(make_pair(-d[v], v));
				}
			}
		}
		long long sum = 0;
		for (int i = 0; i < V; i++) sum += d[i];
		return sum;
	}
};

// ------ Main ------ //
int n, a;
int main() {
	cin >> n; Graph2 g(n);
	for(int i = 0; i < n; i++) {
		for(int j = 0; j < n; j++) {
			cin >> a;
			if(a != -1) g.add1(i, j, a);
		}
	}
	cout << g.minspan() << endl;
}

C++: #include<iostream>
#include<queue>
#include<algorithm>
using namespace std;
typedef pair<int,int> P;
typedef pair<int,P> T;

class uft {//union-find tree
public:
    int parent[101];
    int size[101];
    int root(int x);
    bool same(int x,int y);
    void unite(int x,int y);
};
int uft::root(int x){
    if(parent[x]==x){
        return x;
    }
    return parent[x] = root(parent[x]);
}
bool uft::same(int x,int y){
    if(root(x)==root(y)){
        return true;
    }else{
        return false;
    }
}
void uft::unite(int x,int y){
    x=root(x);
    y=root(y);
    if(x == y) return;
    parent[x] = y;
    size[y] = size[y]+size[x];
}

int solv(priority_queue<T, vector<T>, greater<T> > q,int N){
    uft u;
    for(int i=0;i<N;i++){
        u.parent[i]=i;
        u.size[i]=1;
    }
    int ret=0;
    while(u.size[u.root(0)] != N && !q.empty()){
        T edge = q.top(); q.pop();
        if(u.root((edge.second).first) != u.root((edge.second).second)){
            u.unite((edge.second).first,(edge.second).second);
            ret += edge.first;
        }
    }
    return ret;
}

int main(){
    int N;
    priority_queue<T, vector<T>, greater<T> > q;
    cin>>N;
    for(int i=0;i<N;i++){
        for(int j=0;j<N;j++){
            int a;
            cin>>a;
            if(a!=-1 && i<j){
                T d = T(a,P(i,j));
                q.push(d);
            }
        }
    }
    int ans=solv(q,N);
    cout<<ans<<endl;
    return 0;
}

C++: #include<iostream>


using namespace std;
int n;
int M[100][100];

int prim() {
	int u;
	int color[100];
	int d[100];
	int p[100];
	int mincost;
	for (int i = 0;i < n;i++) {
		color[i] = 0;
		d[i] =2001;
		p[i] = -1;
	}
	d[0] = 0;
	

	while (true) {
		mincost = 2001;
		for (int i = 0;i < n;i++) {
			if (color[i] != 2 && d[i] < mincost) {
				mincost = d[i];
				u = i;
			}
		}
		if (mincost== 2001) {
			break;
		}

		color[u] = 2;
		for (int v = 0;v < n;v++) {
			if (color[v] != 2 && M[u][v] != -1) {
				if (M[u][v] < d[v]) {
					d[v] = M[u][v];
					p[v] = u;
					color[v] = 1;
				}
			}
		}
	}
	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;
		}
	}
	cout << prim() << endl;

	return 0;
}