Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

type ElementKey int
type Node struct {
	key    ElementKey
	parent *Node
	rank   int
}

func newNode(key ElementKey) *Node {
	node := Node{}
	node.key = key
	node.parent = &node
	node.rank = 0

	return &node
}

func union(x, y *Node) {
	xSet := findSet(x)
	ySet := findSet(y)
	link(xSet, ySet)
}

func link(x, y *Node) {
	if x.rank > y.rank {
		y.parent = x
	} else {
		x.parent = y

		if x.rank == y.rank {
			y.rank++
		}
	}
}

func findSet(x *Node) *Node {
	if x != x.parent {
		x.parent = findSet(x.parent)
	}
	return x.parent
}

func newDisjointSets(nSets int) *DisjointSets {
	sets := DisjointSets{}
	sets.keyToNode = map[ElementKey]*Node{}
	for i := 0; i < nSets; i++ {
		key := ElementKey(i)
		sets.keyToNode[key] = newNode(key)
	}
	return &sets
}

type DisjointSets struct {
	keyToNode map[ElementKey]*Node
}

func (sets *DisjointSets) union(x, y ElementKey) {
	xNode := sets.keyToNode[x]
	yNode := sets.keyToNode[y]
	union(xNode, yNode)
}

func (sets DisjointSets) findSet(x ElementKey) *Node {
	xNode := sets.keyToNode[x]
	return findSet(xNode)
}

type Edge struct {
	i, j   int
	weight int
}

func main() {
	scanner.Split(bufio.ScanWords)

	n := nextInt()
	sets := newDisjointSets(n)
	edges := []Edge{}
	a := make([][]int, n)
	for i := 0; i < n; i++ {
		a[i] = make([]int, n)
		for j := 0; j < n; j++ {
			a[i][j] = nextInt()
			if a[i][j] >= 0 {
				e := Edge{
					i:      i,
					j:      j,
					weight: a[i][j],
				}
				edges = append(edges, e)
			}
		}
	}
	sort.Slice(edges, func(i, j int) bool { return edges[i].weight < edges[j].weight })
	totalWeight := 0
	for _, edge := range edges {
		i := ElementKey(edge.i)
		j := ElementKey(edge.j)
		if sets.findSet(i) != sets.findSet(j) {
			totalWeight += edge.weight
			sets.union(i, j)
		}
	}
	fmt.Println(totalWeight)
}

C++: #include <cstdio>
#include <climits>
#include <vector>

int w[100][100];
std::vector<int> mst;
std::vector<int> others;
typedef std::vector<int>::iterator itr;

int minimum(){
    if(others.size()<=0)return 0;
    int candiW=INT_MAX;
    itr candiI;
    for(itr i=mst.begin();i!=mst.end();i++){
        for(itr j=others.begin();j!=others.end();j++){
            if(w[*i][*j]<candiW){
                candiW = w[*i][*j];
                candiI = j;
            }
        }
    }
    mst.push_back(*candiI);
    others.erase(candiI);
    return candiW + minimum();
}

int main(){
    int i, j, n;
    scanf("%d",&n);
    for(i=0;i<n;i++){
        for(j=0;j<n;j++){
            scanf("%d",&w[i][j]);
            if(w[i][j]==-1)w[i][j]=INT_MAX;
        }
    }
    mst.push_back(0);
    for(i=1;i<n;i++)others.push_back(i);
    printf("%d\n",minimum());
    return 0;
}

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

typedef long long ll;

struct Edge {
    ll s,t;
    ll w;
    bool operator < (const Edge& tgt) const {
	return w > tgt.w;
    }
};

ll Prim(const vector<vector<Edge>>& G) {
    const ll V = G.size();
    priority_queue<Edge> edges;
    
    vector<int> used(V,0);
    edges.push( Edge{-1,0,0} );
    ll ans = 0;
    while( edges.size() ) {
	Edge e = edges.top(); edges.pop();

	if( used[e.t] == 1 ) continue;
	used[e.t] = 1;
	ans += e.w;

	for(auto& nxt : G[e.t]) {
	    if( used[nxt.t] == 1 ) continue;
	    edges.push(nxt);
	}
    }
    return ans;
}


int main () {

    ll V; cin >> V;
    vector<vector<Edge>> G(V);
    for(int i=0; i<V; i++) {
	for(int j=0; j<V; j++) {
	    ll w; cin >> w;
	    if( w != -1 ) {
		G[i].push_back( Edge{i,j,w} );
	    }
	}
    }
    cout << Prim(G) << endl;
    return 0;
}

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;
}