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<iostream>
#include<algorithm>
using namespace std;
int p[101],r[101];
void makeset(int x){
    for(int i=0;i<x;++i){
        p[i]=i;
        r[i]=0;
    }
}
int root(int a){
    if(a!=p[a])
        p[a]=root(p[a]);
    return p[a];
}
bool same(int a,int b){
    return (root(a)==root(b));
}
void link(int a,int b){
    if(r[a]>r[b]){
        p[b]=a;
    }
    else{
        p[a]=b;
        if(r[a]==r[b]){
            r[b]++;
        }
    }
}
void unite(int a,int b){
    link(root(a),root(b));
}
struct Edges{int cost,src,dst;}edges[10010];
bool operator<(const Edges &x,const Edges &y){return x.cost<y.cost;}
int main(){
    int N,M=0;
    int A[101][101];
    cin>>N;
    makeset(N);
    for(int r=0;r<N;++r){
        for(int c=0;c<N;++c){
            cin>>A[r][c];
        }
        for(int c=r+1;c<N;++c){
            if(A[r][c]>-1){
                edges[M].cost=A[r][c];
                edges[M].src=r;
                edges[M].dst=c;
                M+=1;
            }
        }
    }
    sort(edges,edges+M);
    int total=0,num=0,flag=0;
    for(int i=0;i<M;++i){
        if(same(edges[i].src,edges[i].dst)){continue;}
        else{
            unite(edges[i].src,edges[i].dst);
            total+=edges[i].cost;
            num++;
        }
        if(num==N-1){
            flag=1;
            break;
        }
    }
    if(flag){
        cout<<total<<endl;
    }
}

C++: #include <bits/stdc++.h>
#define MOD 1000000007
#define INF 1<<30
#define rep(i,n) for(ll i=0; i<(n); i++)
#define REP(i,a,b) for(ll i=(a); i<(b); i++)
#define RREP(i,a,b) for(ll i=(a); i>=(b); i--)
#define all(x) (x).begin(),(x).end()
using namespace std;
typedef long long ll;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef pair<ll, ll> p;
typedef vector<pair<ll, ll>> vpl;

vvl m(105, vl(105,INF));
ll n;

ll prim(){
    ll u, minv=INF;
    vl visit(105,0);
    vl d(105,INF);
    vl p(105,-1);

    d[0] = 0;
    while(1){
        minv = INF;
        rep(i,n){
            if(d[i] < minv && visit[i] == 0){
                minv = d[i];
                u = i;
            }
        }
        if(minv == INF) break;
        visit[u] = 1;
        rep(i,n){
            if(visit[i] == 0 && m[u][i] != INF){
                if(d[i] > m[u][i]){
                    d[i] = m[u][i];
                    p[i] = u;
                }
            }
        }
    }
    ll sum=0;
    rep(i,n){
        if(p[i] != -1) sum += d[i];
    }
    return sum;
}

int main(){
    ll t;
    cin >> n;
    rep(i,n){
        rep(j,n){
            cin >> t;
            m[i][j] = (t==-1) ? INF : t;
        }
    }

    cout << prim() << "\n";

    return 0;
}

C++: #include<iostream>
#include<vector>
#include<utility>
using namespace std;
enum COLOR{WHITE, GRAY, BLACK};

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] = 1e8;
        }
    }
    vector<COLOR> COL(n, WHITE);
    COL[0] = GRAY;
    int sum = 0;
    while (1) {
        pair<int, int> min_bridge = make_pair(0, 0);
        for (int i = 0; i < n; i++) {
            if (COL[i] != GRAY) continue;
            bool flag = true;
            for (int j = 0; j < n; j++) {
                if (COL[j] != WHITE) continue;
                flag = false;
                if (G[i][j] < G[min_bridge.first][min_bridge.second]) {
                    min_bridge.first  = i;
                    min_bridge.second = j;
                }
            }
            if (flag) COL[i] = BLACK;
        }
        if (min_bridge.first == 0 && min_bridge.second == 0) break;
        COL[min_bridge.second] = GRAY;
        sum += G[min_bridge.first][min_bridge.second];
    }
cout << sum << endl;
    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
#define SZ(x) ((int)(x).size())
#define rep(i,n) for(int i=0;i<(n);++i)
#define fore(i,a) for(auto &i:a)
#define min(...) min({__VA_ARGS__})
#define max(...) max({__VA_ARGS__})
#define Yes cout << "Yes" << endl;
#define No cout << "No" << endl;
#define ALL(x) (x).begin(),(x).end()
signed _main(); signed main() { cin.tie(0); ios::sync_with_stdio(false); _main(); }
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; }
template<class T> using v = vector<T>;
template<class T> using vv = vector<v<T>>;
const int MOD=1e9+7;
const long long INF = 1LL << 60;
using P=pair<int,int>;


signed _main(){
  int n; cin >> n;
  vv<int> M(n,v<int>(n));
  rep(i,n)rep(j,n) cin >> M[i][j];
  v<int> color(n,0),d(n,INF),p(n,-1);
  d[0]=0;//TとV-Tをつなぐ辺で最も重みが小さいものの大きさ
  int u,minv,sum=0;

  while(1){
    minv=INF;
    u=-1;//確定した頂点
    rep(i,n){
      if(minv > d[i] && color[i] != 2){
        u=i;
        minv = d[i];
      }
    }
    if(u == -1) break;//更新無し
    color[u]=2;
    rep(v,n){
      if(color[v]!=2 && M[u][v]!=-1){
        if(d[v]>M[u][v]){
          d[v]=M[u][v];
          p[v]=u;
          color[v]=1;
        }
      }
    }
  }

  rep(i,n){
    if(p[i]!=-1) sum+=M[i][p[i]];
  }

  cout << sum << endl;

  return 0;
}