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>
using namespace std;

static const int MAX = 100;
static const int INFTY = (1 << 21);

int n,M[MAX][MAX];

int prim() {
	int u,minv;
	int color[MAX] = {}, d[MAX], p[MAX];
	for (int i = 0;i < n;i++) {
		d[i] = INFTY;
		p[i] = -1;
	}
	d[0] = 0;
	while (1) {
		minv = INFTY;
		u = -1;
		for (int i = 0;i < n;i++) {
			if (minv > d[i] && color[i] != 2) {
				u = i;
				minv = d[i];
			}
		}
		if (u == -1) {
			break;
		}
		color[u] = 2;
		for (int v = 0;v < n;v++) {
			if (color[v] != 2 && M[u][v] != INFTY) {
				if (d[v] > M[u][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;
	int e;
	for (int i = 0;i < n;i++) {
		for (int j = 0;j < n;j++) {
			cin >> e;
			if (e == -1) {
				M[i][j] = INFTY;
			}
			else {
				M[i][j] = e;
			}
			
		}
	}
	cout << prim() << endl;
	return 0;

}

C++: #include <bits/stdc++.h>
using namespace std;
typedef pair<int, int> P;
const int INF = 1e9;
const int MAX_V = 1000;

int cost[MAX_V][MAX_V];
bool used[MAX_V];
int n;

int prim(){
    int res = 0;
    priority_queue<P, vector<P>, greater<P> > q;
    memset(used, false, sizeof(used));
    q.push(P(0, 0));
    while(!q.empty()){
        P p = q.top(); q.pop();
        int v = p.second;
        int d = p.first;
        if(used[v]) continue;
        used[v] = true;
        res += d;
        for(int i = 0; i < n; i++){
            q.push(P(cost[v][i], i));
        }
    }
    return res;
}

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

/* verified
https://onlinejudge.u-aizu.ac.jp/#/courses/lesson/1/ALDS1/12/ALDS1_12_A
*/

C++: #include <bits/stdc++.h>
using namespace std;

struct UF{
   vector<int> par;
   vector<int> sz;
   UF(){}
   UF(int n){
       par.resize(n);
       sz.resize(n, 1);
       for(int i = 0; i < n; i++) par[i] = i;
   }
   int find(int x){
       if(x == par[x]) return x;
       return par[x] = find(par[x]);
   }
   void unite(int x, int y){
       x = find(x); y = find(y);
       if(x == y) return;
       if(sz[x] < sz[y]) swap(x, y);
       sz[x] += sz[y];
       par[y] = x;
   }
   bool same(int x, int y){
       return find(x) == find(y);
   }
};

struct edge{
    int u, v, cost;
    bool operator<(const edge &e) const{
        return cost < e.cost;
    }
};

int n, m;
vector<edge> es; 

int kruskal(){
    int res = 0;    
    UF uf(n);
    sort(es.begin(), es.end());
    for(int i = 0; i < m; i++){
        edge e = es[i];
        if(!uf.same(e.u, e.v)){
            uf.unite(e.u, e.v);
            res += e.cost;
        }
    }
    return res;
}

int main(){
    cin >> n >> m;
    for(int i = 0; i < m; i++){
        int u, v, w;
        cin >> u >> v >> w;
        //u--; v--;
        es.push_back({u, v, w});
    }
    cout << kruskal() << endl;
}

/* verified
https://onlinejudge.u-aizu.ac.jp/#/courses/library/5/GRL/2/GRL_2_A
*/

C++: #include <bits/stdc++.h>
using namespace std;

typedef long long ll;

struct Edge {
    ll s,t;
    ll w;
    bool operator < (const Edge& tgt) const {
	return w > tgt.w;
    }
    void dump(){
	cout << s << " --> " << t << " : " << w << endl; 
    }
};

ll Prim(vector<vector<Edge>>& G) {
    const ll V = G.size();
    priority_queue<Edge> edges;
    
    vector<int> used(V,0);
    //??????0??????????????????
    used[0] = 1;
    for(auto& e : G[0]) edges.push(e);

    ll ans = 0;
    while( !edges.empty() ) {
	Edge e = edges.top(); edges.pop();
	if( used[e.t] == 1 ) continue;
	used[e.t] = 1;
	ans += e.w;
	assert(used[e.s == 1]);
	for(auto& nxt : G[e.t]) {
	    if( used[nxt.t] == 1 ) continue;
	    edges.push(nxt);
	}
    }
    return ans;
}


int main () {

    ll V;
    cin >> V;
    vector<vector<Edge>> G(V);

    for(int i=0; i<V; i++) {
	for(int j=0; j<V; j++) {
	    ll w; cin >> w;
	    if( w != -1 and w != 0) {
		G[i].push_back( Edge{i,j,w} );
		G[j].push_back( Edge{j,i,w} );
//		cout << i << " <--> " << j << " : " << w << endl; 
	    }
	}
    }

    ll ans = Prim(G);
    cout << ans << endl;
        
    return 0;
}