Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"sort"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

type ElementKey int
type Node struct {
	key    ElementKey
	parent *Node
	rank   int
}

func newNode(key ElementKey) *Node {
	node := Node{}
	node.key = key
	node.parent = &node
	node.rank = 0

	return &node
}

func union(x, y *Node) {
	xSet := findSet(x)
	ySet := findSet(y)
	link(xSet, ySet)
}

func link(x, y *Node) {
	if x.rank > y.rank {
		y.parent = x
	} else {
		x.parent = y

		if x.rank == y.rank {
			y.rank++
		}
	}
}

func findSet(x *Node) *Node {
	if x != x.parent {
		x.parent = findSet(x.parent)
	}
	return x.parent
}

func newDisjointSets(nSets int) *DisjointSets {
	sets := DisjointSets{}
	sets.keyToNode = map[ElementKey]*Node{}
	for i := 0; i < nSets; i++ {
		key := ElementKey(i)
		sets.keyToNode[key] = newNode(key)
	}
	return &sets
}

type DisjointSets struct {
	keyToNode map[ElementKey]*Node
}

func (sets *DisjointSets) union(x, y ElementKey) {
	xNode := sets.keyToNode[x]
	yNode := sets.keyToNode[y]
	union(xNode, yNode)
}

func (sets DisjointSets) findSet(x ElementKey) *Node {
	xNode := sets.keyToNode[x]
	return findSet(xNode)
}

type Edge struct {
	i, j   int
	weight int
}

func main() {
	scanner.Split(bufio.ScanWords)

	n := nextInt()
	sets := newDisjointSets(n)
	edges := []Edge{}
	a := make([][]int, n)
	for i := 0; i < n; i++ {
		a[i] = make([]int, n)
		for j := 0; j < n; j++ {
			a[i][j] = nextInt()
			if a[i][j] >= 0 {
				e := Edge{
					i:      i,
					j:      j,
					weight: a[i][j],
				}
				edges = append(edges, e)
			}
		}
	}
	sort.Slice(edges, func(i, j int) bool { return edges[i].weight < edges[j].weight })
	totalWeight := 0
	for _, edge := range edges {
		i := ElementKey(edge.i)
		j := ElementKey(edge.j)
		if sets.findSet(i) != sets.findSet(j) {
			totalWeight += edge.weight
			sets.union(i, j)
		}
	}
	fmt.Println(totalWeight)
}

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);
    edges.push( Edge{-1,0,0} );
    ll ans = 0;
    while( edges.size() ) {
	Edge e = edges.top(); edges.pop();

//	assert(used[e.s] == 1);
	if( used[e.t] == 1 ) continue;
	used[e.t] = 1;
//	e.dump();
	ans += e.w;

	for(auto& nxt : G[e.t]) {
	    if( used[nxt.t] == 1 ) continue;
	    edges.push(nxt);
	}
    }
    for(auto& e : used) assert(e == 1);

    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 ) {
		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;
}

C++: #include <stdio.h>
#define Max 100
#define Maxi 2147483647
#define W  0
#define X  1
#define Z  2

int n,m[Max][Max];
int prim(){
  int i,u,minv;
  int d[Max],p[Max],c[Max];

  for(i=0;i<n;i++){
    d[i] = Maxi;
    p[i] = -1;
    c[i] = W;
  }

  d[0] = 0;
  while(1){
    minv = Maxi;
    u = -1;
    for(i = 0;i < n; i++){
      if(minv > d[i] && c[i] != Z){
        u = i;
        minv = d[i];
      }
    }
    if( u == -1) break;
    c[u] = Z;
    for(i = 0;i<n;i++){
      if(c[i]!= Z && m[u][i]!= Maxi){
        if(d[i] > m[u][i]){
          d[i] = m[u][i];
          p[i] = u;
          c[i] = X;
        }
      }
    }
  }
  int sum = 0;
  for(i = 0; i < n; i++){
    if(p[i]!= -1){
      sum += m[i][p[i]];
    }
  }
  return sum;
}

int main(){

  scanf("%d",&n);
  int i,j,e;
  for(i = 0;i<n;i++){
    for(j = 0;j<n;j++){
      scanf("%d",&e);
      if(e == -1){
        m[i][j] = Maxi;
      }
      else m[i][j] = e;
    }
  }
  printf("%d\n", prim());
  return 0;

}

C++: #include <bits/stdc++.h>
using namespace std;
using Int = int_fast64_t;

class edge{
public:
	Int cost;
	size_t from, to;
	edge(Int c, size_t f, size_t t){
		cost = c;
		from = f;
		to = t;
	}
};
typedef vector<edge> edges;
typedef vector<edges> graph;
class unionfind{ // 0-indexed
public:
	vector<int> parent;
	vector<size_t> size;
	size_t num_of_disjointsets;
	unionfind(size_t n){
		parent.resize(n, -1);
		size.resize(n, 1);
		num_of_disjointsets = n;
	}

	size_t find(size_t x){
		if(parent[x] == -1) return x;
		return parent[x] = find(parent[x]);
	}

	size_t unite(size_t x, size_t y){
		x = find(x);
		y = find(y);
		if(x == y) return -1;
		if(size[x] < size[y]){
			parent[x] = y;
			size[y] += size[x];
			size[x] = 0;
			--num_of_disjointsets;
			return y;
		}else{
			parent[y] = x;
			size[x] += size[y];
			size[y] = 0;
			return x;
			--num_of_disjointsets;
		}
	}

	bool united(size_t x, size_t y){
		return find(x) == find(y);
	}
};
bool operator < (const edge &e, const edge &f){
	return e.cost != f.cost ? e.cost > f.cost : e.from != f.from ? e.from < f.from : e.to < f.to;
}
vector<edge> kruskal(graph &g){
	priority_queue<edge> pq;
	for(size_t i=0; i<g.size(); ++i)
		for(size_t j=0; j<g[i].size(); ++j)
			if(i < g[i][j].to)
				pq.push(g[i][j]);
	unionfind uf(g.size());
	vector<edge> res;
	while(!pq.empty()){
		edge e = pq.top(); pq.pop();
		if(!uf.united(e.from, e.to)){
			uf.unite(e.from, e.to);
			res.push_back(e);
		}
		if(uf.num_of_disjointsets == 1) break;
	}
	return res;
}

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	Int n; cin >> n;
	graph g(n);
	for(Int i=0; i<n; ++i)
		for(Int j=0; j<n; ++j){
			Int a; cin >> a;
			if(a != -1){
				g[i].push_back(edge(a, i, j));
				g[j].push_back(edge(a, j, i));
			}
		}
	vector<edge> mst = kruskal(g);
	Int ans = 0;
	for(auto i:mst) ans += i.cost;
	cout << ans << "\n";
}

C++: #include <iostream>
#include <vector>
#include <utility>
#include <algorithm>
using namespace std;
int n;
int A[110][110];
int P[10010] = {0};
void init(int N){
	for(int i=0;i<=N;++i) P[i] = i;
}

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

bool is_same_set(int a,int b){
	return root(a) == root(b);
}

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


int main(){
	while(cin >> n && n>0){
		init(n);
		for(int i=0;i<n;i++){
			for(int j=0;j<n;j++){
				cin >> A[i][j];
			}
		}
		int cost[10000],src[10000],dst[10000];
		vector< pair<int,int> > edges;
		for(int r=0;r<n;r++){
			for(int c=r+1;c<n;c++){
				if(A[r][c] > -1){
					cost[M] = A[r][c];
					src[M] = r;
					dst[M] = c;
					//cout << cost[M] << endl;
					pair<int,int> edge = make_pair(cost[M],M);
					edges.push_back(edge);
					M++;
				}
			}
		}
		sort(edges.begin(),edges.end());
		long long  sum = 0;
		for(int i=0;i<M;i++){
			int n1 = src[edges[i].second],n2 = dst[edges[i].second];
			if(!is_same_set(n1,n2)){
				unite(n1,n2);
				sum += edges[i].first;
			}
		}
		cout << sum << endl;
	}
}

#if 0
所要時間は45分程度。クラスカル法を回す部分の実装に少し苦戦した。
#endif