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<iostream>
using namespace std;
#define MAX 100
#define INFTY 2001
#define WHITE 0
#define GRAY 1
#define BLACK 2

int M[MAX][MAX], d[MAX];

void prim(int n){
  int color[MAX], u;

  // 初期化
  for(int i=0; i<n; i++){
    color[i] = WHITE;
    d[i] = INFTY;
  }
  d[0] = 0;
  //p[0] = -1;

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

    if(mincost == INFTY) break;

    color[u] = BLACK;

    for(int v=0; v<n; v++){
      if(color[v] != BLACK && M[u][v] != INFTY){
        if(M[u][v] < d[v]){
          d[v] = M[u][v];
          //p[v] = u;
          color[v] = GRAY;
        }
      }
    }
  }
}

int main(void){
  int n;
  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] = INFTY;
    }
  }

  prim(n);

  // 最小全域木の辺の重みの総和を計算
  int u = 0;
  int d_sum = 0;
  for(int i=0; i<n; i++){
    if(true){
      d_sum += d[i];
    }
  }
  cout << d_sum << endl;
  return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<climits>
using namespace std;

#define N 105
int n;

int g[N][N];
int mini;
int weight[N];
bool us[N];

int prim(){
  int v;
  int ans=0;
  weight[1]=0;
  
  while(true){
    v=-1;
   
    for(int i=1;i<=n;i++){
      if(us[i]!=true && (v==-1 ||weight[i]<weight[v]))v=i;
    }

    if(v==-1)break;
    us[v]=true;
    ans+=weight[v];

    for(int i=1;i<=n;i++){
      if(weight[i]>g[v][i])weight[i]=g[v][i];
      else weight[i]=weight[i];
    }
  }
  return ans;
}


int main(){

  //load n
  cin >>n;

  //load data
  for(int i=1;i<=n;i++){
    weight[i]=1000;
    us[i]=false;
    for(int j=1;j<=n;j++){
      scanf("%d",&g[i][j]);
      if(g[i][j]== -1)g[i][j]=1000;
      //    cout <<g[i][j]<<" ";
    }
    //    cout <<endl;
  }




  //print
  cout <<prim()<<endl;

  return 0;

}

C++: #include <algorithm>
#include <iostream>
#include <set>
#include <vector>
#define FOR(i,k,n) for (int (i)=(k); (i)<(n); ++(i))
#define REP(i,n) FOR(i,0,n)
#define pb push_back
#define mp make_pair
#define fst first
#define snd second
#define all(v) begin(v), end(v)
using namespace std;
typedef pair<int,int> PII;

int P[100]; //union_find

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

bool same(int x, int y){
    return root(P[x]) == root(P[y]);
}

void unite(int x, int y){
    P[root(x)] = root(y);
}

int main(){
    int n;
    cin >> n;
    int g[n][n];
    vector<pair<int,PII> > V;
    REP(i,n){
        REP(j,n){
            cin >> g[i][j];
            if(i<j && g[i][j] >= 0) V.pb(mp(g[i][j],mp(i,j)));
        }
    }
    sort(all(V));
    REP(i,n){
        P[i] = i;
    }
    int wsum = 0, count = 0;
    REP(i,n*n){
        int x = V[i].snd.fst;
        int y = V[i].snd.snd;
        if(same(x,y)) continue;
        unite(x,y);
        wsum += V[i].fst;
        count++;
        if(count == n-1) break;
    }

    cout <<wsum<<endl;

    return 0;
}

C++: #include <cstdio>
#include <cctype>
#include <vector>
#include <queue>
using namespace std;

#define si static inline
#define gcu getchar_unlocked
si int in() {
	int c, n = 0;
	bool minus = false;
	if ((c = gcu()) == '-') minus = true; else ungetc(c, stdin);
	while ((c = gcu()) >= '0' && c <= '9') n = 10 * n + (c - '0');
	//return n; }
	return minus ? -n : n; }
si void scan(char *s) {while (!isspace(*s++ = gcu()));}
#define pcu putchar_unlocked
#define svo si void out
template <typename T>
svo(T n) {
	static char buf[20];
	char *p = buf;
	//if(n < 0) pcu('-'), n *= -1;
	if (!n) *p++ = '0';
	else while (n) *p++ = n % 10 + '0', n /= 10;
	while (p != buf) pcu(*--p); }
svo(const char *s){while(*s)pcu(*s++);}
svo(char *s){while(*s)pcu(*s++);}
svo(char c){pcu(c);}
template <typename T>
//svo(vector<T> &v){for(T &x:v)out(' '),out(x);out('\n');}
svo(vector<T> &v){for(T &x:v)out(&x == &v[0]?"":" "),out(x);out('\n');}
template <typename head, typename... tail>
svo(head&& h, tail&&... t){out(h);out(move(t)...);}
#undef svo
#undef si

#define v vector
struct graph {
	struct node {int n, w;};
	v<v<node>> nodes;

	void read(int n) {
		nodes.resize(n);
		for (auto &x: nodes) {
			gcu();
			for (int i = 0, w; i < n; i++)
				if ((w = in()) != -1)
					x.push_back({i, w});
		}
	}

	int mst_prim() {
		int w = 0;
		v<bool> visited(nodes.size());
		priority_queue<node> q;
		q.push({0, 0});
		while (!q.empty()) {
			node n = q.top();
			q.pop();
			if (visited[n.n])
				continue;
			w += n.w;
			visited[n.n] = true;
			for (node &i: nodes[n.n])
				q.push(i);
		}

		return w;
	}
};

bool operator < (const graph::node &a, const graph::node &b) {
	return a.w > b.w;
}

int main() {
	graph g;
	g.read(in());
	out(g.mst_prim(), '\n');
}

C++: #include<iostream>
using namespace std;
static const int MAX=100;
static const int INFTY=(1<<21);
static const int WHITE=0;
static const int GRAY=1;
static const int BLACK=2;

int n,m[MAX][MAX];

int prim(){
  int u,minv;
  int d[MAX],p[MAX],color[MAX];

  for(int i=0;i<n;i++){
    d[i]=INFTY;
    p[i]=-1;
    color[i]=WHITE;
  }

  d[0]=0;

  while(1){
    minv=INFTY;
    u=-1;
    for(int i=0;i<n;i++){
      if(minv>d[i]&&color[i]!=BLACK){
	u=i;
	minv=d[i];
      }
    }
    if(u==-1) break;
    color[u]=BLACK;
    for(int v=0;v<n;v++){
      if(color[v]!=BLACK&&m[u][v]!=INFTY){
	if(d[v]>m[u][v]){
	  d[v]=m[u][v];
	  p[v]=u;
	  color[v]=GRAY;
	}
      }
    }
  }
  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==-1)?INFTY:e;
    }
  }
  cout<<prim()<<endl;

  return 0;
}