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 <bits/stdc++.h>
using namespace std;
#define rep(i,n)for(int i=0;i<(n);i++)
using ll = long long;
using P = pair<int, int>;

const int INF = 1001001001;
vector<vector<int>> cost;
vector<int> mincost;
vector<bool> used;
int V;

int prim(){
    mincost.assign(V,INF);
    used.assign(V,0);
    mincost[0] = 0;
    int res = 0;
    while(true){
        int v = -1;
        for(int u = 0; u < V; ++u){
            if(!used[u] && (v == -1 || mincost[u] < mincost[v])) v = u;
        }
        if(v == -1) break;//completed
        used[v] = 1;
        res += mincost[v];
        for(int u = 0; u < V; ++u){
            if(cost[v][u] != -1)mincost[u] = min(mincost[u], cost[v][u]);//確定した頂点から行ける頂点について、最小コストを更新
        }
    }
    return res;
}


int main(){
    cin >> V;
    cost.assign(V,vector<int>(V));
    rep(i,V)rep(j,V) cin >> cost[i][j];
    int ans = prim();
    cout << ans << endl;
}

C++: #include <iostream>
#include <vector>
#include <queue>
#include <tuple>

using namespace std;

class UnionFindTree {
private:
    vector<int32_t> parent, rank;

    int32_t find(int32_t x) {
        if (parent[x] == x) {
            return x;
        } else {
            int32_t p = find(parent[x]);
            parent[x] = p;
            return p;
        }
    }

public:
    UnionFindTree(int32_t n) {
        parent.resize(n);
        rank.resize(n);

        for (int32_t i = 0; i < n; i++) {
            parent[i] = i;
            rank[i] = 0;
        }
    }

    void unite(int32_t x, int32_t y) {
        x = find(x);
        y = find(y);

        if (x == y) return;

        if (rank[x] < rank[y]) {
            parent[x] = y;
        } else {
            parent[y] = x;
            if (rank[x] == rank[y]) rank[x]++;
        }
    }

    bool same(int32_t x, int32_t y) {
        return find(x) == find(y);
    }
};

int main() {
    int32_t n;
    cin >> n;

    UnionFindTree uf(n);
    priority_queue<tuple<int32_t, int32_t, int32_t>, vector<tuple<int32_t, int32_t, int32_t>>, greater<tuple<int32_t, int32_t, int32_t>>> E;

    for (int32_t i = 0; i < n; i++) {
        for (int32_t j = 0; j < n; j++) {
            int32_t w;
            cin >> w;

            if (w > -1) {
                E.push(make_tuple(w, i, j));
            }
        }
    }

    int32_t sum = 0;
    while (!E.empty()) {
        int32_t w, u, v;
        tie(w, u, v) = E.top();
        E.pop();
        if (!uf.same(u, v)) {
            sum += w;
            uf.unite(u, v);
        }
    }

    cout << sum << endl;

    return 0;
}