Go: package main

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

//util
var sc = bufio.NewScanner(os.Stdin)

func nextStr() string {
	sc.Scan()
	return sc.Text()
}

func nextInt() int {
	sc.Scan()
	i, e := strconv.Atoi(sc.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func printList(list []int) {
	n := len(list)
	buf := bufio.NewWriter(os.Stdout)
	for _, v := range list[0 : n-1] {
		buf.Write([]byte(fmt.Sprintf("%d ", v)))
	}
	buf.Write([]byte(fmt.Sprintf("%d\n", list[n-1])))
	buf.Flush()
}

func max(a, b int) int {
	if a > b {
		return a
	} else {
		return b
	}
}

func min(a, b int) int {
	if a > b {
		return b
	} else {
		return a
	}
}

//solve
const (
	MAX   = 100
	INFTY = (1 << 21)
	WHITE = 0
	GRAY  = 1
	BLACK = 2
)

//V: グラフの頂点全体の集合
//T: MSTに属する頂点の集合
var (
	n     int
	M     [MAX][MAX]int //重みを記録する隣接行列
	color [MAX]int
	p     [MAX]int //MSTにおける頂点の親を記録
	d     [MAX]int //Tに属する頂点と、V−Tに属する頂点を繋ぐ辺の中で、重みが最小のものを記録
)

func prim() int {
	var u, minv int
	//初期化
	for i := 0; i < n; i++ {
		d[i] = INFTY
		p[i] = -1
		color[i] = WHITE
	}

	d[0] = 0

	for {
		//頂点uを決めていく
		//つまり、Tに属する頂点と、V−Tに属する頂点を繋ぐ辺の中で、重みが最小のものを決める
		minv = INFTY
		u = -1
		for i := 0; i < n; i++ {
			//未訪問かつ、辺の重みが最小となるuを選ぶ
			if minv > d[i] && color[i] != BLACK {
				u = i
				minv = d[i]
			}
		}

		if u == -1 {
			break
		}

		color[u] = BLACK
		for v := 0; v < n; v++ {
			//未訪問かつ、辺が存在するとき
			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
				}
			}
		}
	}

	sum := 0
	for i := 0; i < n; i++ {
		if p[i] != -1 {
			sum += M[i][p[i]]
		}
	}
	return sum
}

func main() {
	sc.Split(bufio.ScanWords)
	n = nextInt()

	for i := 0; i < n; i++ {

		for j := 0; j < n; j++ {
			tmp := nextInt()
			if tmp == -1 {
				tmp = INFTY
			}
			M[i][j] = tmp
		}

	}

	fmt.Println(prim())

}

C++: #include <algorithm>
#include <bitset>
#include <cassert>
#include <cmath>
#include <ctime>
#include <cstring>
#include <functional>
#include <iostream>
#include <iomanip>
#include <limits>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <regex>
#include <vector>

#define fix(n)          cout<<fixed<<setprecision(n)
#define rep(i,n)        for (int i = 0; i < (n); ++i)
#define all(a)          (a).begin(), (a).end()
#define sort(a)         sort(all(a))
#define uniq(a)         sort(a);(a).erase(unique(all(a)), (a).end())
#define reverse(a)      reverse(all(a))
#define ctos(c)         string(1, (c))
#define out(d)          cout << (d)
#define outl(d)         std::cout<<(d)<<"\n"
#define YES()           cout << "YES" << endl
#define NO()            cout << "NO" << endl
#define Yes()           cout << "Yes" << endl
#define No()            cout << "No" << endl
#define ceil(x, y)      ((x + y - 1) / (y))
#define debug(x)        cerr << #x << ": " << (x) << '\n'
#define debug2(x, y)    cerr << #x << ": " << (x) << ", " << #y << ": " << (y) << '\n'
#define debug3(x, y, z) cerr << #x << ": " << (x) << ", " << #y << ": " << (y) << ", " << #z << ": " << (z) << '\n'
#define dbg(v)          for (size_t _ = 0; _ < v.size(); ++_){ cerr << #v << "[" << _ << "] : " << v[_] << '\n'; }
#define pb              push_back
#define fst             first
#define int             long long
#define INF             __LONG_LONG_MAX__

using namespace std;
using ll = long long;
using ld = long double;
using P = pair<ll,ll>;

const ll MOD = 1000000007; // 10^9 + 7
const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};

void solve() {
  const ll MAX = 100;

  ll n; cin >> n;
  ll mat[MAX][MAX];
  rep(i, n)rep(j, n) cin >> mat[i][j];

  vector<bool> mst(n, false);
  ll ans = 0;
  mst[0] = true;
  priority_queue<tuple<ll, ll, ll>> pq;
  for (int i = 1; i < n; i++) {
    if (mat[0][i] != -1) pq.emplace(-mat[0][i], 0, i);
  }

  ll w, f, t;
  rep(i, n-1) {
    while(1) {
      tie(w, f, t) = pq.top(); pq.pop();
      if (mst[f] && !mst[t]) break;
    }
    // debug3(-w, f, t);
    ans += mat[f][t];
    mst[t] = true;

    for (int j = 0; j < n; j++) {
      if (t != j && mat[t][j] != -1) pq.emplace(-mat[t][j], t, j);
    }
  }
  outl(ans);
}

signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  srand((unsigned)time(NULL));
  fix(12);

  solve();
}

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 M = 0;
		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