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

template<typename T>
struct Kruskal{
    struct edge{
        int from,to,used; T cost;
        edge(int from,int to,T cost):
            from(from),to(to),cost(cost),used(0){}
        bool operator<(const edge &e) const{
            if (cost!=e.cost) return cost<e.cost;
            else if (from!=e.from) return from<e.from;
            else return to<e.to;
        }
    };
    int num;
    vector<int> par,rank;
    vector<edge> es;
    Kruskal(int n):num(n),par(n),rank(n,1){
        iota(par.begin(),par.end(),0);
    }
    int root(int x){
        if (par[x]==x) return x;
        return par[x]=root(par[x]);
    }
    bool merge(int x, int y){
        x=root(x),y=root(y);
        if (x==y) return false;
        if (rank[x]<rank[y]) swap(x,y);
        par[y]=x;
        rank[x]+=rank[y];
        --num;
        return true;
    }
    bool same(int x, int y){return root(x)==root(y);}
    int size(int x){return rank[root(x)];}
    int count(){return num;}
    void add_edge(int u,int v,T c){
        es.emplace_back(u,v,c);
    }
    T build(){
        sort(es.begin(),es.end());
        T res=0;
        for (auto &e:es){
            if (!same(e.from,e.to)){
                res+=e.cost;
                merge(e.from,e.to);
                e.used=1;
            }
        }
        return res;
    }
    vector<int> used_es(T &c){
        int E=es.size();
        map<edge,int> m;
        for (int i=0;i<E;++i) m[es[i]]=i;
        c=build();
        vector<int> list(E);
        for (int i=0;i<E;++i) list[m[es[i]]]=es[i].used;
        return list;
    }
};

// https://onlinejudge.u-aizu.ac.jp/problems/ALDS1_12_A

void ALDS1_12_A(){
    int n; cin >> n;
    Kruskal<int> K(n);
    vector<vector<int>> a(n,vector<int>(n));
    for (int i=0;i<n;++i){
        for (int j=0;j<n;++j){
            cin >> a[i][j];
            if (j>i&&~a[i][j]) K.add_edge(i,j,a[i][j]);
        }
    }
    cout << K.build() << '\n';
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    ALDS1_12_A();
}

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;
    }
    void dump(){
	cout << s << " --> " << t << " : " << w << endl; 
    }
};

ll Prim(const vector<vector<Edge>>& G) {
    const ll V = G.size();
    priority_queue<Edge> edges;
    
    vector<int> used(V,0);
    //??????0??????????????????
    used[0] = 1;
    for(auto& e : G[0]) edges.push(e);

    ll ans = 0;
    while( !edges.empty() ) {
	Edge e = edges.top(); edges.pop();

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

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

    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 and w != 0) {
		G[i].push_back( Edge{i,j,w} );
//		G[j].push_back( Edge{j,i,w} );
//		cout << i << " <--> " << j << " : " << w << endl; 
	    }
	}
    }

    ll ans = Prim(G);
    cout << ans << endl;
        
    return 0;
}

C++: #include <iostream> 
#include<vector>
#include<algorithm>
#include<map>
#include<iomanip>
#include<set>
#include<queue>
#include<deque>
#include<iomanip>
#include<sstream>
#include<cmath>
#include<bitset>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define req(i,n) for(int i = 1;i <=  n; i++)
#define rrep(i,n) for(int i = n -1;i >= 0;i--)
#define ALL(obj) begin(obj), end(obj)
typedef long long int ll;
typedef long double ld;
const ll INF = (1 << 29);
ll h, m, n; string s;
const ll MOD= 1000000007;
const ld PI = acos(-1);
ll sum = 0;
template<typename A, size_t N, typename T>
void Fill(A(&array)[N], const T & val) {
    std::fill((T*)array, (T*)(array + N), val);
}int a[110][101];
int prim() {
    vector<int> d(110, INF), p(110, -1),co(110,0);
    d[0] = 0; int u, miv = 0;
    while (1) {
        miv = INF;
        u = -1;
        rep(i, n) {
            if (miv > d[i] && co[i] != 2) {
                u = i; miv = d[i];
            }
        }
        if (u == -1) break;
        co[u] = 2;
        rep(v, n) {
            if (co[v] != 2 && a[u][v] != INF) {
                if (d[v] > a[u][v]) {
                    d[v] = a[u][v];
                    p[v] = u; co[v] = 1;
                }
            }
        }
    }
    rep(i, n) {
        if (p[i] != -1) sum += a[i][p[i]];
    }return sum;
}
int main() {
    cin >> n; 
    rep(i, n) {
        rep(j, n) {
            cin >> a[i][j];
            if (a[i][j] == -1) {
                a[i][j] = INF;
            }
        }
    }
    cout << prim() << endl;
}

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

#define MAX 100
#define INF 2097152
#define WHITE 0
#define GRAY 1
#define BLACK 2

int n, M[MAX][MAX];

int main(void){

	int i,j,f;
	int n,u,v,minv,d[MAX],p[MAX],color[MAX],sum=0;
	scanf("%d",&n);
  
	for(i=0;i<n;i++){
		for(j=0;j<n;j++){
			scanf("%d",&f);
			if(f == -1){
				M[i][j] = INF;
			}else{
				M[i][j] = f;
			}
		}
	}


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

	d[0] = 0;

	while(1){
		minv = INF;
		u = -1;
		for(i=0;i<n;i++){
			if(minv>d[i] && color[i] != BLACK){
				u = i;
				minv = d[i];
			}
		}
		if(u == -1) break;
		color[u] = BLACK;
		for(v=0;v<n;v++){
			if(color[v] != BLACK && M[u][v] != INF){
				if(d[v] > M[u][v]){
					d[v] = M[u][v];
					p[v] = u;
					color[v] = GRAY;
				}
			}
		}
	}

	for(i=0;i<n;i++){
		if(p[i] != -1){ 
			sum+= M[i][p[i]];
		}
	}

	printf("%d\n",sum);


return 0;

}