Go: package main

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

// I/O
type Scanner struct {
	sc *bufio.Scanner
}

func NewScanner() *Scanner {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	sc.Buffer(make([]byte, 1024), int(1e+9))
	return &Scanner{sc}
}

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

func (s *Scanner) nextInt() int {
	i, e := strconv.Atoi(s.nextStr())
	if e != nil {
		panic(e)
	}
	return i
}

func (s *Scanner) nextFloat() float64 {
	f, e := strconv.ParseFloat(s.nextStr(), 64)
	if e != nil {
		panic(e)
	}
	return f
}

func (s *Scanner) nextRuneSlice() []rune {
	return []rune(s.nextStr())
}

func (s *Scanner) nextIntSlice(n int) []int {
	res := make([]int, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextInt()
	}
	return res
}

func (s *Scanner) nextFloatSlice(n int) []float64 {
	res := make([]float64, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextFloat()
	}
	return res
}

// Arithmetic
func max(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m < i {
			m = i
		}
	}
	return m
}

func min(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m > i {
			m = i
		}
	}
	return m
}

func abs(x int) int {
	if x > 0 {
		return x
	}
	return -x
}

func pow(x, y int) int {
	res := 1
	for i := 0; i < y; i++ {
		res *= x
	}
	return res
}

func ceil(a, b int) int {
	if a%b == 0 {
		return a / b
	} else {
		return a/b + 1
	}
}

// Sort
type RuneSlice []rune

func (a RuneSlice) Len() int           { return len(a) }
func (a RuneSlice) Less(i, j int) bool { return a[i] < a[j] }
func (a RuneSlice) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }

const INF = 1 << 60

// Main
func main() {
	sc := NewScanner()
	wtr := bufio.NewWriter(os.Stdout)
	N := sc.nextInt()
	G := make([][]int, N)
	for i := 0; i < N; i++ {
		G[i] = make([]int, N)
		for j := 0; j < N; j++ {
			G[i][j] = sc.nextInt()
		}
	}
	d := make([]int, N)
	for i := 0; i < N; i++ {
		d[i] = INF
	}
	seen := make([]bool, N)
	d[0] = 0
	ans := 0
	for {
		cur := 0
		minCost := INF
		for i := 0; i < N; i++ {
			if !seen[i] && d[i] < minCost {
				minCost = d[i]
				cur = i
			}
		}
		if minCost == INF {
			break
		}
		seen[cur] = true
		ans += d[cur]
		for i := 0; i < N; i++ {
			cost := G[cur][i]
			if cost != -1 && cost < d[i] {
				d[i] = cost
			}
		}
	}

	fmt.Fprintln(wtr, ans)
	wtr.Flush()
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
using vll = vector<ll>;
using vi = vector<int>;
using vb = vector<bool>;
using pii = pair<int, int>;

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

//pow(llpow,modpow)
template <class T>
ll llpow(ll x, T n)
{
    ll ans = 1;
    if (x == 0)
        ans = 0;
    while (n)
    {
        if (n & 1)
            ans *= x;
        x *= x;
        n >>= 1;
    }
    return ans;
}
long long modpow(long long a, long long n, long long mod)
{
    long long res = 1;
    while (n > 0)
    {
        if (n & 1)
            res = res * a % mod;
        a = a * a % mod;
        n >>= 1;
    }
    return res;
}
//最大公約数
template <class T>
inline T gcd(T x, T y)
{
    if (y == 0)
        return x;
    else
    {
        return gcd(y, x % y);
    }
}
//最小公倍数
template <class T>
inline T lcm(T x, T y) { return x / gcd(x, y) * y; }
//逆元
long long modinv(long long a, long long m)
{
    long long b = m, u = 1, v = 0;
    while (b)
    {
        long long t = a / b;
        a -= t * b;
        swap(a, b);
        u -= t * v;
        swap(u, v);
    }
    u %= m;
    if (u < 0)
        u += m;
    return u;
}

#define rep(i, begin_i, end_i) for (ll i = (ll)begin_i; i < (ll)end_i; i++)

long long INF = 1LL << 60;
int main()
{
    int n;
    cin >> n;
    vector<vi> a(n,vector<int> (n));
    rep(i, 0, n)
    {
        rep(j, 0, n)
        {
            cin >> a[i][j];
            if(a[i][j]==-1)a[i][j]=1<<30;
        }
    }
    ll res = 0;
    set<int> rem,visited;
    visited.insert(0);
    rep(i,1,n)rem.insert(i);
    while(!rem.empty()){
        int minv=1<<30,mini=-1;
        for(int prev:visited){
            for(int next:rem){
                if(a[prev][next]<minv){
                    minv=a[prev][next];
                    mini=next;
                }
            }
        }
        res+=minv;
        rem.erase(mini);
        visited.insert(mini);
    }
    cout<<res<<endl;
    return 0;
}

C++: /* minimum_spanning_tree_prim.cpp
    プリム法で最小全域木とそのコストを求める（隣接行列 向き）
    計算量: O(|E|log|V|)

    verified: AOJ ALDS1_12_A Minimum Spanning Tree
        http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ALDS1_12_A&lang=jp
 */

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

vector<vector<int>> G;  // 隣接行列での表現

/* Prim : プリム法で minimum spanning tree を求める構造体
    入力: 隣接行列でのグラフ G
    最小全域木の重みの総和: sum
    計算量: O(|V|^2)
*/
struct Prim {
    const int INF = 1e9;
    int sum;              // MSTの重みの総和
    int n;                // 頂点数
    vector<int> mincost;  // 既に確定した頂点からの最小コスト（確定済みから行けない時は INF）
    vector<bool> used;    // 既に確定したかどうか
    Prim(vector<vector<int>> const& Graph) {
        init(Graph);
    }
    void init(vector<vector<int>> const& Graph) {
        n = (int)Graph.size();
        mincost.assign(n, INF);
        used.assign(n, false);
        sum = 0;
        mincost[0] = 0;  // 始め頂点0からスタートさせる
        while (true) {
            int v = -1;
            for (int u = 0; u < n; u++) {  // コストが最小で行ける頂点 v を探す
                if (!used[u] && (v == -1 || mincost[u] < mincost[v])) v = u;
            }
            if (v == -1) break;  // MST ができたので終了
            used[v] = true;
            sum += mincost[v];
            for (int u = 0; u < n; 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
                if (Graph[v][u] != -1) mincost[u] = min(mincost[u], Graph[v][u]);
            }
        }
    }
};

int main() {
    int n;
    cin >> n;
    G.assign(n, vector<int>(n));
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            cin >> G[i][j];
        }
    }
    Prim prim(G);
    cout << prim.sum << endl;
    return 0;
}

C++: #include <iostream>

using namespace std;

#define INT_MIN     (-2147483647 - 1) /* minimum (signed) int value */
#define INT_MAX       2147483647    /* maximum (signed) int value */
static const int MAX = 100;
static const int WHITE = 0;
static const int GRAY = 1;
static const int BLACK = 2;

int n, m[MAX][MAX];

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

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

	d[0] = 0;

	while (1)
	{
		minv = INT_MAX;
		u = -1;
		for (int i = 0; i < n; ++i)
		{
			if (d[i] < minv && color[i] != BLACK)
			{
				u = i;
				minv = d[i];
			}
		}

		if (-1 == u) 
			break;
		color[u] = BLACK;

		for (int i = 0; i < n; ++i)
		{
			if (m[u][i] < d[i] && color[i] != BLACK)
			{
				d[i] = m[u][i];
				p[i] = u;
				color[i] = GRAY;
			}
		}
	}

	int sum = 0;
	for (int i = 0; i < n; ++i)
	{
		if (p[i] != -1) 
			sum += m[p[i]][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) ? INT_MAX : e;
		}
	}
	cout << prim() << endl;
}