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 12:20:56
*/
//   @URL : http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ALDS1_12_A
#include<bits/stdc++.h>
#include<algorithm>
#include <cstdlib>
#define INF 0x3f3f3f3f
using namespace std;
typedef long long LL;
const int MAXN = 1e2+10;

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

int prim()
{
	int minv,u,v;
	memset(p,-1,sizeof p);
	memset(d,INF,sizeof d);

	d[1] = 0;

	while(1)
	{
		minv = INF;
		u = -1;
		for(int i = 1; i <= n; ++i)
		{
			if(minv > d[i] && !color[i])
				minv = d[i],u = i;
		}
		if(u == -1)	break;
		color[u] = 1;
		for(int v = 1; v <= n; ++v)
		{
			if(!color[v] && a[u][v] != INF)
			{
				if(d[v] > a[u][v])
				{
					d[v] = a[u][v];
					p[v] = u;
				}
			}
		}
	}
	
	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<iostream>
#define MAX 100
#define MAXNUM 2001
using namespace std;

int n, M[MAX][MAX];
int u, minv, total = 0;
int w[MAX], p[MAX], color[MAX] = {};

void prim() {
	for (int i = 0; i < n; i++) {
		w[i] = MAXNUM;
		p[i] = -1;
	}
	w[0] = 0;

	while (1) {
		minv = MAXNUM;
		u = -1;
		for (int i = 0; i < n; i++) {
			if (w[i] < minv && color[i] != 2) {
				u = i;
				minv = w[i];
			}
		}
		if (u == -1) break;
		color[u] = 2;
		for (int i = 0; i < n; i++) {
			if (color[i] != 2 && M[u][i] != MAXNUM) {
				if (M[u][i] < w[i]) {
					w[i] = M[u][i];
					p[i] = u;
					color[i] = 1;
				}
			}
		}
	}
}

int main() {
	cin >> n;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			cin >> M[i][j];
			if (M[i][j] == -1) {
				M[i][j] = MAXNUM;
			}
		}
	}
	prim();
	for (int i = 0; i < n; i++) {
		if (p[i] != -1) total += M[i][p[i]];
	}
	cout << total << endl;
}

C++: #include <iostream>
using namespace std;
#define INF 1000000

int main(){
  long n;
  long G[100][100],d[100];
  long minc,mi,i;
  bool flg[i];
  long sum=0;
  
  cin >> n;
  for (i=0;i<n;i++){
    for (int j=0;j<n;j++){
      cin >> G[i][j];
      if (G[i][j]==-1) G[i][j]=INF;
    }
    d[i]=INF;
    flg[i]=false;
  }

  d[0]=0;
  while (true){
    minc=INF;
    for (i=0;i<n;i++){
      if (!flg[i]&&d[i]<minc){
	minc=d[i];
	mi=i;
      }
    }
    if (minc==INF) break;
    flg[mi]=true;
    for (i=0;i<n;i++){
      if (G[mi][i]==INF)continue;
      if (!flg[i]&&d[i]>G[mi][i]){
	d[i]=G[mi][i];
	//cout << d[i];
      }
    }
  }
  
  for (i=0;i<n;i++){
    sum+=d[i];
  }
  cout << sum<<endl;
  
  return 0;
}

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

int x[10000][10000];
int MIN[10000];
int MINX = 0;
int MINZ = 10000000;

int main() {
	memset(MIN, 127, sizeof(MIN));
	int n, a;
	cin >> n;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			cin >> a;
			x[i][j] = a;
		}
	}
	for(int i=0;i<n;i++){
	    for(int j=0;j<n;j++){
	        if(x[i][j]!=-1){
	            MIN[i]=min(x[i][j],MIN[i]);
	        }
	    }
	}
	for(int i=0;i<n;i++){
	    MINX=0;
	    for(int j=0;j<n;j++){
	        MINX += MIN[j];
	    }
	    MINZ=min(MINZ,MINX);
	}
	cout<<MINZ<<endl;
	return 0;
}

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

// 総数を1000000007で割った余り
const long long mod = 1e9 + 7;

using ll  = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
#define ull unsigned long long
#define ld long double
#define vi vector<int>
#define vll vector<ll>
#define vc vector<char>
#define vs vector<string>
#define vpii vector<pii>
#define vpll vector<pll>

#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; i++)
#define rep1(i, n) for (int i = 1, i##_len = (n); i <= i##_len; i++)
#define repr(i, n) for (int i = ((int)(n)-1); i >= 0; i--)
#define rep1r(i, n) for (int i = ((int)(n)); i >= 1; i--)

#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()

#define SORT(v, n) sort(v, v + n);
// #define SORT(v, n) stable_sort(v, v + n);
#define VSORT(v) sort(v.begin(), v.end());
#define RSORT(x) sort(rall(x));
#define pb push_back
#define mp make_pair

#define PI (acos(-1))
#define EPS (1e-7)

ull gcd(ull a, ull b) { return b ? gcd(b, a % b) : a; }
ull lcm(ull a, ull b) { return a / gcd(a, b) * b; }

const int MAX   = 100;
const int INF   = (1 << 21);
const int white = 0;
const int gray  = 1;
const int black = 2;

int n, m[MAX][MAX];

int prim() {
    int u, minV;
    int d[MAX], p[MAX], color[MAX];
    rep(i, n) {
        d[i]     = INF;
        p[i]     = -1;
        color[i] = white;
    }

    d[0] = 0;
    while (1) {
        minV = INF;
        u    = -1;
        rep(i, n) {
            if (minV > d[i] && color[i] != black) {
                u    = i;
                minV = d[i];
            }
        }
        if (u == -1) break;
        color[u] = black;
        rep(v, n) {
            if (color[v] != black && m[u][v] != INF) {
                if (d[v] > m[u][v]) {
                    d[v]     = m[u][v];
                    p[v]     = u;
                    color[v] = gray;
                }
            }
        }
    }
    int sum = 0;
    rep(i, n) {
        if (p[i] != -1) sum += m[i][p[i]];
    }
    return sum;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    // cout << fixed << setprecision(5);

    cin >> n;
    rep(i, n) {
        rep(j, n) {
            int e;
            cin >> e;
            m[i][j] = (e == -1) ? INF : e;
        }
    }
    cout << prim() << endl;

    return 0;
}