Go: package main

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

const BIG_COST = 3000

const (
	ST_NOT_YET = iota
	ST_JOINED
)

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)

	nNodes := scanInt(sc)
	costs := make([][]int, nNodes)

	for r := 0; r < nNodes; r++ {
		costs[r] = make([]int, nNodes)
		for c := 0; c < nNodes; c++ {
			costs[r][c] = scanInt(sc)
		}
	}

	states := make([]int, nNodes)
	minCosts := make([]int, nNodes)

	for i := 1; i < nNodes; i++ {
		minCosts[i] = BIG_COST
	}

	for {
		var u int
		minCost := BIG_COST

		for i := 0; i < nNodes; i++ {
			if states[i] != ST_JOINED && minCosts[i] < minCost {
				minCost = minCosts[i]
				u = i
			}
		}

		// 全てのノードが MST に JOIN できたので終了。
		if minCost == BIG_COST {
			break
		}

		states[u] = ST_JOINED

		for v := 0; v < nNodes; v++ {
			cost := costs[u][v]

			// MST内の u と MST外の v との辺の重みをそれぞれ minCosts[v] として
			// 保存しておく。このうち辺の重みが最小となる v が次のループで MST に JOIN する。
			if states[v] != ST_JOINED && cost != -1 && cost < minCosts[v] {
				minCosts[v] = cost
			}
		}
	}

	sum := 0
	for _, cost := range minCosts {
		sum += cost
	}
	fmt.Println(sum)
}

func scanInt(sc *bufio.Scanner) int {
	sc.Scan()
	v, _ := strconv.Atoi(sc.Text())
	return v
}

C++: #include <cstdio>
#include <vector>
#include <algorithm>
#include <queue>

using namespace std;

template <class T>
using gp_queue=priority_queue<T, deque<T>, less<T>>;

using Weight=int;
using DArray=vector<Weight>;
using DMatrix=vector<DArray>;

static const Weight INF=1<<29;

struct Edge {
    size_t src, dst;
    Weight weight;
    Edge() {}
    Edge(size_t src, size_t dst, Weight weight=1):
        src(src), dst(dst), weight(weight)
    {}
};

bool operator<(const Edge &e, const Edge &f) {
    if (e.weight != f.weight) {
        return e.weight > f.weight;
    } else {
        return e.src!=f.src? e.src<f.src : e.dst<f.dst;
    }
}

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

void join(Graph &g, size_t s, size_t d, Weight w=1) {
    // non-directed
    g[s].push_back(Edge(s, d, w));
    g[d].push_back(Edge(d, s, w));
}

void connect(Graph &g, size_t s, size_t d, Weight w=1) {
    // directed
    g[s].push_back(Edge(s, d, w));
}

struct UnionFindTree {
    size_t n;
    vector<int> tree;
    UnionFindTree(size_t n): n(n) {
        tree = vector<int>(n, -1);
    }
    int find_root(size_t v) {
        if (tree[v] < 0) return v;
        return tree[v] = find_root(tree[v]);
    }
    bool unite(size_t x, size_t y) {
        x = find_root(x);
        y = find_root(y);
        if (x == y) return false;
        if (rank(x) < rank(y)) swap(x, y);
        tree[x] += tree[y];
        tree[y] = x;
        return true;
    }
    bool has_same_root(size_t x, size_t y) {
        return find_root(x) == find_root(y);
    }
    size_t rank(size_t v) {
        return -tree[find_root(v)];
    }
};

pair<Weight, Edges> minimum_spanning_forest(const Graph &g) {
    /* Kruskal */
    size_t V=g.size();
    UnionFindTree tree(V);
    gp_queue<Edge> q;
    for (size_t u=0; u<V; ++u)
        for (const Edge &e: g[u])
            if (u < e.dst) q.push(e);

    Weight total=0;
    Edges F;
    while (F.size()<V-1 && !q.empty()) {
        Edge e=q.top(); q.pop();
        if (tree.unite(e.src, e.dst)) {
            F.push_back(e);
            total += e.weight;
        }
    }

    return make_pair(total, F);
}

int main() {
    size_t n;
    scanf("%zu", &n);

    Graph g(n);
    for (size_t i=0; i<n; ++i)
        for (size_t j=0; j<n; ++j) {
            Weight a;
            scanf("%d", &a);

            if (a > -1)
                connect(g, i, j, a);
        }

    printf("%d\n", minimum_spanning_forest(g).first);
    return 0;
}

C++: // clang-format off
#include <bits/stdc++.h>
#define int long long
#define main signed main()
#define loop(i, a, n) for (int i = (a); i < (n); i++)
#define rep(i, n) loop(i, 0, n)
#define forever while (true)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define prec(n) fixed << setprecision(n)
template<typename A> using V = std::vector<A>;
template<typename A> using F = std::function<A>;
template<typename A, typename B> using P = std::pair<A, B>;
using pii = P<int, int>;
using vi = V<int>;
using vd = V<double>;
using vs = V<std::string>;
using vpii = V<pii>;
using vvi = V<vi>;
using vvpii = V<vpii>;
constexpr int INF = sizeof(int) == sizeof(long long) ? 1000000000000000000LL : 1000000000;
constexpr int MOD = 1000000007;
constexpr double PI = acos(-1);
template<typename A, typename B> bool cmin(A &a, const B &b) { return a > b ? (a = b, true) : false; }
template<typename A, typename B> bool cmax(A &a, const B &b) { return a < b ? (a = b, true) : false; }
template<typename T> std::istream &operator>>(std::istream &is, std::vector<T> &v) { for (T &x : v) is >> x; return is; }
using namespace std;
// clang-format on

using Weight = int;
struct Edge {
  int src, dst;
  Weight weight;
  Edge(const int &s = 0, const int &d = 0, const Weight &w = 0) : src(s), dst(d), weight(w) {}
};

using Edges = std::vector<Edge>;
using Array = std::vector<Weight>;
using Matrix = std::vector<Array>;

class Graph {
  std::vector<Edges> g;

public:
  Graph(const int &size = 0) : g(size) {}

  void addArc(const int &src, const int &dst, const Weight &w = 1) { g[src].emplace_back(src, dst, w); }

  void addEdge(const int &node1, const int &node2, const Weight &w = 1) {
    addArc(node1, node2, w);
    addArc(node2, node1, w);
  }

  const Edges &operator[](const int &i) const { return g[i]; }

  Graph toRootedTree(int r);
  std::pair<std::vector<Weight>, std::vector<int>> dijkstra(const int &src, const int &inf);
  Matrix warshallFloyd(const int &inf);
  std::pair<Weight, Edges> prim(const int &r);
  template<typename UnionFind> std::pair<Weight, Edges> kruskal();
  Weight solveTSP(const int &inf);
};

std::pair<Weight, Edges> Graph::prim(const int &r = 0) {
  struct comp {
    bool operator()(const Edge &e, const Edge &f) const { return e.weight > f.weight; }
  };
  Edges tree;
  Weight total = 0;
  std::vector<int> vs(g.size());
  std::priority_queue<Edge, std::vector<Edge>, comp> q;
  q.emplace(-1, r, 0);
  while (q.size()) {
    Edge e = q.top();
    q.pop();
    if (vs[e.dst]) continue;
    vs[e.dst] = true;
    total += e.weight;
    if (e.src != -1) tree.emplace_back(e);
    for (auto &f : g[e.dst]) {
      if (!vs[f.dst]) q.emplace(f);
    }
  }
  return {total, tree};
}

main {
  int n;
  cin >> n;
  Graph g(n);
  rep(i, n) rep(j, n) {
    int a;
    cin >> a;
    if (a >= 0) g.addArc(i, j, a);
  }
  cout << g.prim().first << endl;
}

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

enum color{WHITE, GRAY, BLACK};

int calc(vector<vector<int> >& G, vector<color>& state){
    int i_min = 0;
    int j_min = 0;
    for (int i = 0; i < G.size(); i++) {
        if (state[i] == WHITE || state[i] == BLACK) {
            continue;
        }
        bool is_black = true;
        for (int j = 0; j < G[i].size(); j++) {
            if (state[j] == WHITE){
                is_black = false;
                if (G[i][j] < G[i_min][j_min]) {
                    i_min = i;
                    j_min = j;
                }
            }
        }
        if (is_black) state[i] = BLACK;
    }
    state[j_min] = GRAY;
    return G[i_min][j_min];
}

int Prim(vector<vector<int> >& G) {
    vector<color> state(G.size(), WHITE);
    state[0] = GRAY;
    int ret = 0;

    for (int i = 0; i < G.size() - 1; i++) {
        ret += calc(G, state);
    }
    return ret;
}

int main(){
    int n;
    cin >> n;
    vector<vector<int> > G(n, vector<int>(n));
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            cin >> G[i][j];
            if (G[i][j] == -1) G[i][j] = 1e6;
        }
    }

    cout << Prim(G) << endl;

    return 0;
}