Go: package main

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

func getScanner(fp *os.File) *bufio.Scanner {
	scanner := bufio.NewScanner(fp)
	scanner.Split(bufio.ScanWords)
	scanner.Buffer(make([]byte, 500001), 500000)
	return scanner
}

func getNextString(scanner *bufio.Scanner) string {
	scanner.Scan()
	return scanner.Text()
}

func getNextInt(scanner *bufio.Scanner) int {
	i, _ := strconv.Atoi(getNextString(scanner))
	return i
}

func main() {
	fp := os.Stdin
	if len(os.Args) > 1 {
		fp, _ = os.Open(os.Args[1])
	}

	scanner := getScanner(fp)
	writer := bufio.NewWriter(os.Stdout)

	n := getNextInt(scanner)

	g := make([][]int, n)
	ans := make([][]int, n)

	inf := 1 << 30
	for i := 0; i < n; i++ {
		g[i] = make([]int, n)
		ans[i] = make([]int, n)
		for ii := 0; ii < n; ii++ {
			g[i][ii] = getNextInt(scanner)
			if g[i][ii] == -1 {
				g[i][ii] = inf
			}
			ans[i][ii] = -1
		}
	}

	im := make(IntMap, 1)

	im.increment(1)

	for len(im) < n {
		min := inf
		u := -1
		v := -1
		for i := range im {
			for ii := 0; ii < n; ii++ {
				if im.get(ii) != 0 {
					continue
				}
				if min > g[i][ii] {
					min = g[i][ii]
					u = i
					v = ii
				}
			}
		}
		im.increment(v)
		ans[u][v] = min
		ans[v][u] = min
	}

	sum := 0
	for i := 0; i < n-1; i++ {
		for ii := i + 1; ii < n; ii++ {
			if ans[i][ii] != -1 {
				sum += ans[i][ii]
			}
		}
	}

	fmt.Fprintln(writer, sum)
	writer.Flush()
}

// IntMap ...
type IntMap map[int]int

func (m IntMap) get(key int) int {
	if _, ok := m[key]; ok {
		return m[key]
	}
	return 0
}
func (m IntMap) increment(key int) {
	if _, ok := m[key]; ok == false {
		m[key] = 0
	}
	m[key]++
}

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

const int white = 0;
const int gray = 1;
const int black = 2;
const int Max_n = 100;
const int Max_a = 2000;

int M[Max_n][Max_n];
int color[Max_n], d[Max_n], p[Max_n];
int n;

int prim(){
    for(int i=n; i<n; i++){
        color[i] = white;
        d[i] = Max_a + 1;
    }

    d[0] = 0;
    p[0] = -1;

    while(1){
        int mincost = Max_a + 1;
        int u;
        for(int i=0; i<n; i++){
            if(color[i] != black && d[i] < mincost){
                mincost = d[i];
                u = i;
            }
        }
        if(mincost == Max_a + 1){
            break;
        }

        color[u] = black;

        for(int v=0; v<n; v++){
            if(color[v] != black && M[u][v] != -1){
                if(M[u][v] < d[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++){
            cin >> M[i][j];
        }
    }

    cout << prim() << endl;


    return 0;
}

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

#define rep(i, a, b) for (int i = (a); i < (int)(b); i++)
#define rrep(i, a, b) for (int i = (a); i >= (int)(b); i--)
#define all(x) (x).begin(), (x).end()
using i32 = int32_t;
using i64 = int64_t;
using f32 = float;
using f64 = double;
using P   = pair<int, int>;

template <class T>
bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    } else {
        return false;
    }
}
template <class T>
bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    } else {
        return false;
    }
}

template <class T>
void dump_vec(const vector<T>& v) {
    auto len = v.size();
    rep(i, 0, len) {
        cout << v[i] << (i == (int)len - 1 ? "\n" : " ");
    }
}

struct FastIO {
    FastIO() {
        cin.tie(0);
        ios::sync_with_stdio(false);
        cout << fixed << setprecision(20);
    }
} FASTIO;

//---------------------------------------------------------------------------------------------------

struct Edge {
    int from, to;
    i64 cost;
    Edge() {}
    Edge(int t, i64 c) : from(-1), to(t), cost(c) {}
    Edge(int f, int t, i64 c) : from(f), to(t), cost(c) {}

    bool operator<(const Edge& r) const {
        return cost < r.cost;
    }
    bool operator>(const Edge& r) const {
        return cost > r.cost;
    }
};

using Vertex = vector<Edge>;
using Graph  = vector<Vertex>;

// 無向
void link(Graph& g, int from, int to, int cost = 1) {
    g[from].push_back(Edge(from, to, cost));
    g[to].push_back(Edge(to, from, cost));
    return;
}

// 有向
void connect(Graph& g, int from, int to, int cost = 1) {
    g[from].push_back(Edge(from, to, cost));
    return;
}

pair<i64, Vertex> minimum_spanning_tree(const Graph& g, int start) {
    i64 total_cost = 0;
    Vertex V;
    vector<bool> visited(g.size(), false);

    priority_queue<Edge, vector<Edge>, greater<Edge>> que;
    que.push(Edge(start, start, 0));
    while (!que.empty()) {
        const auto e = que.top();
        que.pop();
        if (visited[e.to]) continue;

        visited[e.to] = true;
        V.push_back(e);
        total_cost += e.cost;
        for (const auto& n : g[e.to]) {
            if (!visited[n.to]) {
                que.push(n);
            }
        }
    }
    return make_pair(total_cost, V);
}

//---------------------------------------------------------------------------------------------------

signed main() {
    int N;
    cin >> N;
    Graph G(N);
    rep(i, 0, N) rep(j, 0, N) {
        int w;
        cin >> w;
        if (w >= 0) {
            connect(G, i, j, w);
        }
    }
    auto ans = minimum_spanning_tree(G, 0);
    cout << ans.first << "\n";
}