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>
using namespace std;
#define N 105
#define INF 1<<21
int n;
int a[N][N];
int d[N],p[N],color[N];              //0.未访问的节点   1.正在访问的节点  2.已经访问过的节点 
int prim(){
	int u ,minv;
	for (int i = 0;i < n;i++){
		d[i] = INF;
		p[i] = -1;
		color[i] = 0; 
	}
	d[0] = 0;
	while (1){
		minv = INF;
		u = -1;
		for (int i = 0;i < n;i++){
			if (minv > d[i] && color[i] != 2){
				u = i;
				minv = d[i]; 
			}
		}
		if ( u == -1) break;
		color[u] = 2;
		for (int v = 0;v < n;v++){
			if (color[v] != 2 && a[u][v] != INF){
				if (d[v] > a[u][v]){
					d[v] = a[u][v];
					p[v] = u;
					color[v] = 1;
				}
			}
		}
	}
	int sum = 0;
	for (int i = 0;i < n;i ++){
		if (p[i] != -1){
			sum = sum + a[i][p[i]];
		}
	}
	return sum;
}

int main()
{
	cin >> n;
	int x;
	for (int i = 0;i < n;i++){
		for (int j = 0;j < n;j++){
			cin >> x;
			if (x == -1){
				a[i][j] = INF;
			}
			else{
				a[i][j] = x;
			}
		}
	}
	cout << prim() << endl;
	return 0;
}

C++: #include<iostream>
#include<algorithm>
using namespace std;

int P[101];

void init(int N)
{
	for (int i=0; i<N; ++i) 
	    P[i] = i;
} 

int root(int a)
{
	if(P[a]==a)   
	   return a;
	else  return(P[a]=root(P[a]));
}
 
bool is_same_set(int a,int b)
{
	return(root(a)==root(b));
}

void unite(int a,int b)
{
	P[root(a)]=root(b);
}

int main()
{
	int n;
	cin >> n;
	init(n);
	int a[101][101];
	for(int i=0;i<n;i++)
	   for(int j=0;j<n;j++)
	       cin >> a[i][j];
	int M=0;
	int cost[10100],src[10100],dst[10100];
	for(int r=0;r<n;r++)
	   for(int c=r+1;c<n;c++)
	      if(a[r][c]>-1)
	      {
	      	cost[M]=a[r][c];
	      	src[M]=r;
	      	dst[M]=c;
	      	M++;
		  }
	pair<int,int> edges[10100];
	for(int i=0;i<M;i++)
	{
		edges[i].first=cost[i];
		edges[i].second=i;
	}
	sort(edges,edges+M);
	int sum=0;
	for(int i=0;i<M;i++)
	{
		if(is_same_set(src[edges[i].second],dst[edges[i].second]))
		  continue;
		unite(src[edges[i].second],dst[edges[i].second]);
		sum+=edges[i].first;
	}
	cout << sum << endl;
}

C++: #include <stdio.h>
#define MAX 100
#define INFTY 100000000
#define WHITE 0
#define BLACK 2
#define GRAY 1

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(){
    scanf("%d",&n);
    for(int i=0; i<n; i++){
        for(int j=0; j<n; j++){
            int e;
            scanf("%d",&e);
            M[i][j] = (e == -1) ? INFTY:e;
            
        }
    }
    printf("%d\n", prim());
    return 0;
}

C++: #if 0

#endif

#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;

int P[10010];
void init(int N) {
  for (int i=1; i<=N; ++i)
    P[i] = i;
}
int root(int a) {
  if (P[a] == a)
    return a;
  return (P[a] = root(P[a]));
}
bool is_same_set(int a, int b) {
  return root(a) == root(b);
}
void unite(int a, int b) {
  P[root(a)] = root(b);
}

int n;

int main() {
  cin >> n;
  init(n);
  vector<pair<int,int>> NC;
  int src[10010]; int dst[10010];
  int ind = 0;
  int c;
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      cin >> c;
      if (i < j && c >= 0) {
        ind++;
        NC.push_back(make_pair(c, ind));
        src[ind] = i; dst[ind] = j;
      }
    }
  }
  sort(NC.begin(), NC.end());
  int ans = 0;
  int cst, id, s, d;
  for (int i = 0; i < ind; i++) {
    cst = NC[i].first; id = NC[i].second;
    s = src[id]; d = dst[id];
    if (!(is_same_set(s, d))) {
      ans += cst;
      unite(s, d);
    }
  }
  cout << ans << endl;
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <queue>
#include <algorithm>
using namespace std;

#define MAX_N 100
#define MAX_EDGE_COST 20000
int Graph[MAX_N+1][MAX_N+1];
int n;

int prim(int initial_vertex) {
  bool visited[n+1];
  for(int i=1;i<=n;i++) visited[i] = false;
  vector<int> vertexes;
  vertexes.push_back(initial_vertex);
  visited[initial_vertex] = true;
  int total_edge_cost = 0;
  while(vertexes.size() < n) {
    // find minimum edge cost in vertexes
    int minimum_edge_cost = MAX_EDGE_COST;
    int next_v;
    for(int i=0;i<vertexes.size();i++) {
      int v = vertexes[i];
      for(int j=1;j<=n;j++) {
        if(!visited[j]) {
          if(Graph[v][j] != -1) {
            if(Graph[v][j] < minimum_edge_cost) {
              minimum_edge_cost = Graph[v][j];
              next_v = j;
            }
          }
        }
      }
    }
    visited[next_v] = true;
    vertexes.push_back(next_v);
    total_edge_cost += minimum_edge_cost;
  }
  return total_edge_cost;
}

int main() {
  scanf("%d",&n);
  for(int i=1;i<=n;i++) {
    for(int j=1;j<=n;j++) {
      scanf("%d ",&Graph[i][j]);
    }
  }
  printf("%d\n",prim(1));
}