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++: /*
* @Author: Samson
* @Date:   2018-06-12 18:24:44
* @Last Modified by:   Samson
* @Last Modified time: 2018-06-13 11:56:23
*/
//   @URL : http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ALDS1_12_A
#include<bits/stdc++.h>
#include<algorithm>
#include <cstdlib>
#define white 0
#define gray 1
#define black 2
#define INF 0x3f3f3f3f
using namespace std;
typedef long long LL;
const int MAXN = 1e3+10;

int p[MAXN],color[MAXN],a[MAXN][MAXN],t,n,d[MAXN];

int prim()
{
	int minv,u,v;
	for(int i = 1; i <= n; ++i)
	{
		d[i] = INF;
		p[i] = -1;
		color[i] = white;
	}
	
	d[0] = 0;

	while(1)
	{
		minv = INF;
		u = -1;
		for(int i = 1; i <= n; ++i)
		{
			if(minv > d[i] && color[i] != black)
				minv = d[i],u = i;
		}
		if(u == -1)	break;
		color[u] = black;
		for(int v = 1; v <= n; ++v)
		{
			if(color[v] != black && a[u][v] != INF)
			{
				if(d[v] > a[u][v])
				{
					d[v] = a[u][v];
					p[v] = u;
					color[v] = gray;
				}
			}
		}
	}
	int ans = 0;
	for(int i = 1; i <= n; ++i)
		if(p[i] != -1)
			ans += a[i][p[i]];
	return ans;
}

int main(void)
{
	ios::sync_with_stdio(false); 
	//cin.tie(0);
	cin>>n;
	for(int i = 1; i <= n; ++i)
	{
		for(int j = 1; j <= n; ++j)
		{
			cin>>t;
			a[i][j] = t==-1?INF:t;
		}
	}
	cout<<prim()<<'\n';
	return 0;
}

C++: #include<bits/stdc++.h>
#define ll long long
#define rep(i, n) for(int i=0;i<(n);++i)
#define per(i, n) for(int i=(n)-1;i>=0;--i)
#define repa(i, n) for(int i=1;i<(n);++i)
#define foreach(i, n) for(auto &i:(n))
#define pii pair<int, int>
#define pll pair<long long, long long>
#define all(x) (x).begin(), (x).end()
#define bit(x) (1ll << (x))
const ll MOD = (ll)1e9+7;
const int INF = (ll)1e9+7;
const ll INFLL = (ll)1e18;
using namespace std;
template<class t>
using vvector = vector<vector<t>>;
template<class t>
using vvvector = vector<vector<vector<t>>>;
template<class t>
using priority_queuer = priority_queue<t, vector<t>, greater<t>>;
template<class t, class u> bool chmax(t &a, u b){if(a<b){a=b;return true;}return false;}
template<class t, class u> bool chmin(t &a, u b){if(a>b){a=b;return true;}return false;}

ll modpow(ll x, ll b){
	ll res = 1;
	while(b){
		if(b&1)res = res * x % MOD;
		x = x * x % MOD;
		b>>=1;
	}
	return res;
}

ll modinv(ll x){
	return modpow(x, MOD-2);
}

bool was_output = false;
template<class t>
void output(t a){
	if(was_output)cout << " ";
	cout << a;
	was_output = true;
}
void outendl(){
	was_output = false;
	cout << endl;
}

class minimum_spanning_tree{
public:
	int n;
	vvector<pll> edges;
	minimum_spanning_tree(int N):n(N){
		edges.resize(n);
	}

	void add_edge(int from, int to, ll cost){
		edges[from].emplace_back(to, cost);
	}

	ll solve(){
		ll res = 0;
		priority_queuer<pll> pq;
		pq.emplace(0, 0);
		vector<bool> used(n, false);
		while(pq.size()){
			pll d = pq.top();
			pq.pop();
			if(used[d.second]){
				continue;
			}
			used[d.second] = true;
			res += d.first;
			foreach(i, edges[d.second]){
				if(used[i.first])continue;
				pq.emplace(i.second, i.first);
			}
		}
		return res;
	}
};


int main(){
	int n;
	cin >> n;
	minimum_spanning_tree msp(n);
	rep(i, n){
		rep(j, n){
			int cost;
			cin >> cost;
			if(cost==-1)continue;
			msp.add_edge(i, j, cost);
		}
	}
	cout << msp.solve() << endl;
	return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <vector>

using namespace std;

struct Wside{
	int vertex;
	int weight;
	Wside(int a, int b){
		vertex=a; weight=b;
	}
};

vector<Wside>* info;
vector<int> scanned;
int n;
int minimum;

bool isScanned(int v){
	for(int i=0,sz=scanned.size(); i<sz; ++i){
		if(v==scanned[i]) return 1;
	}
	return 0;
}

int scan(){
	int temp_min=100000;
	int temp_v = 0;
	for(int i=0, sz=scanned.size(); i<sz; ++i){
		int s=scanned[i];
		for(int j=0, sz2=info[s].size(); j<sz2; ++j){
			if(isScanned(info[s][j].vertex)){
				continue;
			}
			temp_min = temp_min<info[s][j].weight? temp_min: info[s][j].weight;
			temp_v = temp_min<info[s][j].weight? temp_v: info[s][j].vertex;
		}
	}

	minimum += temp_min;
	scanned.push_back(temp_v);
	return 0;
}

int main()
{
	scanf("%d",&n);
	info = new vector<Wside> [n];

	int temp;
	for(int i=0; i<n; ++i){
		for(int j=0; j<n; ++j){
			scanf("%d",&temp);
			if(temp!=-1){
				info[i].push_back(Wside(j, temp));
			}
		}
	}

	scanned.push_back(0);
	while(scanned.size()!=n){
		scan();
	}
	cout << minimum << endl;

	delete[] info;

	return 0;
}

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

int n;
int cost[1000010],src[10010],dst[10010];
int a[110][110];
int P[10010];
int M;
class edges{
public:
    int x;
    int y;
    int costs;
}edges[1000010];
bool comp(class edges a,class edges b)
{
    return a.costs<b.costs;
}
void init(int N) {
    for (int i=1; i<=N; ++i)
        P[i] = i;
}

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);
}

void inits(){
    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;
                M++;
            }
        }
    }
    for(int i=0;i<M;i++){
        edges[i].x=src[i];
        edges[i].y=dst[i];
        edges[i].costs=cost[i];
}
    sort(edges,edges+M,comp);
}
int solve()
{
    int sum=0;
    for(int i=0;i<M;i++){
        if(same(edges[i].x,edges[i].y))
            continue;
        unite(edges[i].x,edges[i].y);
        sum+=edges[i].costs;
    }
    return sum;
}

int main(){
    cin>>n;
    init(n);
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++)
        cin>>a[i][j];
    }
    inits();
    cout<<solve()<<endl;
    return 0;
}