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;
typedef long long ll;
const int INF=2147483647;
const double pi=3.1415926535897932384626;
inline ll read(){
	ll x=0,f=1;
	char ch=getchar();
	while(ch<'0'||ch>'9'){
		if(ch=='-') f=-1;
		ch=getchar();
	}
	while(ch>='0'&&ch<='9'){
		x=(x<<1)+(x<<3)+(ch^48);
		ch=getchar();
	}
	return x*f;
}
const int maxn=1e2+5;
int n,x,ans,minn,m[maxn][maxn],d[maxn],fa[maxn],vis[maxn];
int main(){
	n=read();
	for(int i=1;i<=n;i++)
		for(int j=1;j<=n;j++){
			x=read();
			m[i][j]=(x==-1)?INF:x;
		}
	for(int i=1;i<=n;i++) d[i]=INF;
	d[1]=0;
	int u;
	while(1){
		u=0;
		minn=INF;
		for(int i=1;i<=n;i++)
			if(minn>d[i]&&!vis[i]){
				u=i;
				minn=d[i];
			}
		if(u==0) break;
		vis[u]=1;
		for(int i=1;i<=n;i++)
			if(!vis[i]&&m[u][i]!=INF&&d[i]>m[u][i]){
				d[i]=m[u][i];
				fa[i]=u;
			}	
	}
	for(int i=1;i<=n;i++)
		if(fa[i]) ans+=m[i][fa[i]];
	printf("%d\n",ans);
	return 0;
}

C++: #include<iostream>
using namespace std;
static const int MAX=100;
static const int INFTY=(1<<21);
static const int WHITE=0;
static const int GRAY =1;
static const int BLACK=2;

int n,M[MAX][MAX];

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

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

  while(1){
    minv=INFTY;
    u=-1;
    for(int 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(int v=0;v<n;v++){
      if(color[v]!=BLACK&&M[u][v]!=INFTY){
	if(d[v]>M[u][v]){
	  d[v]=M[u][v];
	  p[v]=u;
	  color[v]=GRAY;
	}
      }
    }
  }
      int sum=0;
      for(int i=0;i<n;i++){
	if(p[i]!=-1) sum+=M[i][p[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) ? INFTY:e;
    }
  }

  cout<<prim()<<endl;

  return 0;
}

C++: #include <bits/stdc++.h>
#define ll long long
#define INF 1000000005
#define MOD 1000000007
#define EPS 1e-10
#define rep(i,n) for(int i=0;i<(int)(n);++i)
#define rrep(i,n) for(int i=(int)(n)-1;i>=0;--i)
#define srep(i,s,t) for(int i=(int)(s);i<(int)(t);++i)
#define each(a,b) for(auto (a): (b))
#define all(v) (v).begin(),(v).end()
#define len(v) (int)(v).size()
#define zip(v) sort(all(v)),v.erase(unique(all(v)),v.end())
#define cmx(x,y) x=max(x,y)
#define cmn(x,y) x=min(x,y)
#define fi first
#define se second
#define pb push_back
#define show(x) cout<<#x<<" = "<<(x)<<endl
#define spair(p) cout<<#p<<": "<<p.fi<<" "<<p.se<<endl
#define svec(v) cout<<#v<<":";rep(kbrni,v.size())cout<<" "<<v[kbrni];cout<<endl
#define sset(s) cout<<#s<<":";each(kbrni,s)cout<<" "<<kbrni;cout<<endl
#define smap(m) cout<<#m<<":";each(kbrni,m)cout<<" {"<<kbrni.first<<":"<<kbrni.second<<"}";cout<<endl

using namespace std;

typedef pair<int,int> P;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<double> vd;
typedef vector<P> vp;
typedef vector<string> vs;

class UF {
private:
    int sz; vector<int> par,nrank;
public:
    UF(){}
    UF(int node_size){ sz = node_size; par.resize(sz),nrank.resize(sz); rep(i,sz){ par[i] = i; nrank[i] = 0; } }
    int find(int x){ if(par[x] == x){ return x; }else{ return par[x] = find(par[x]); } }
    void unite(int x,int y)
    { x = find(x),y = find(y); if(x == y) return;
    	if(nrank[x] < nrank[y]){ par[x] = y; }else{ par[y] = x; if(nrank[x] == nrank[y]) nrank[x]++; } }
    bool same(int x,int y){ return find(x) == find(y); }
};

template<typename T> class Kruskal{
public:
	struct edge{
		int u,v;
		T cost;
		bool operator<(const edge& another) const {
			return cost < another.cost;
		}
	};
	vector<edge> es;
	vector<int> par,rrank;
	int V;
    Kruskal(int node_size) : V{node_size}, par{node_size}, rrank{node_size}{
		// V = node_size;
		// par.resize(node_size),rrank.resize(node_size);
	}
	void add_edge(int u,int v,T cost){
		es.push_back((edge){u,v,cost});
	}
	T make(){
		UF uf(V);
		T res = 0;
		int cnt = 0;
		rep(i,es.size()){
			edge e = es[i];
			if(!uf.same(e.u,e.v)){
				uf.unite(e.u,e.v);
				res += e.cost;
				cnt++;
				if(cnt == V-1){
					break;
				}
			}
		}
		return res;
	}
};


int main()
{
    int n;
    cin >> n;
    Kruskal<int> kr(n);
    rep(i,n){
        rep(j,n){
            int a;
            cin >> a;
            if(i < j){
                kr.add_edge(i,j,a);
            }
        }
    }
    cout << kr.make() << "\n";
}

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

#define MAX_NODE 100
#define INF 1000000000

using namespace std;

struct Edge{
	int to, w;
};

vector <Edge> e[MAX_NODE];

int minw[MAX_NODE];
bool flag[MAX_NODE];

int prim(int n,int start){
	fill(minw,minw+n,INF);
	fill(flag,flag+n,false);
	minw[start]=0;
	int sum=0;
	int from = start;
	while(1){
		for(int i=0;i<e[from].size();i++){
			if(e[from][i].w<minw[e[from][i].to]){
				minw[e[from][i].to]=e[from][i].w;
			}		
		}
		int minimum_e;
		minimum_e=INF;
		for(int i=0;i<n;i++){
			if(minimum_e>minw[i]&&flag[i]==false){
				minimum_e=minw[i];
				from=i;
			}
		}
		if(minimum_e==INF){
			break;
		}
		sum+=minimum_e;
		flag[from]=true;
	}
	return sum;
}

int main(){
	int n,tmp,sum;
	Edge edge;
	cin>>n;
	for(int i=0;i<n;i++){
		for(int j=0;j<n;j++){
			cin>>tmp;
			if(tmp!=-1){
				edge.to=j;
				edge.w=tmp;
				e[i].push_back(edge);
			}
		}
	}
	sum=prim(n,0);
	cout<<sum<<endl;
	return 0;
}