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;
#define int long long   // <-----!!!!!!!!!!!!!!!!!!!

#define rep(i,n) for (int i=0;i<(n);i++)
#define rep2(i,a,b) for (int i=(a);i<(b);i++)
#define rrep(i,n) for (int i=(n)-1;i>=0;i--)
#define rrep2(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define all(a) (a).begin(),(a).end()

typedef long long ll;
typedef pair<int, int> P;
typedef tuple<int, int, int> TUPLE;
typedef vector<int> V;
typedef vector<V> VV;
typedef vector<VV> VVV;

class UnionFind {
private:
    int n;
    vector<int> uni;
public:
    UnionFind(int _n) {
        n = _n;
        uni.clear();
        uni.resize(n, -1);
    }
    int root(int x) {
        if (uni[x] < 0) return x;
        return uni[x] = root(uni[x]);
    }
    bool same(int x, int y) {
        return root(x) == root(y);
    }
    void unite(int x, int y) {
        x = root(x);
        y = root(y);
        if (x == y) return;
        if (uni[x] > uni[y]) swap(x, y);
        uni[x] += uni[y];
        uni[y] = x;
    }
    void print() {
        for (auto x : uni) cout << x << " ";
        cout << endl;
    }
};

struct edge {
    int from, to, cost;
    edge(){}
    edge(int _from, int _to, int _cost) : from(_from), to(_to), cost(_cost) {}
    bool operator< (const edge& e) const {
        return cost == e.cost ? (from == e.from ? to < e.to : from < e.from) : cost < e.cost;
    }
};

class Kruskal {
private:
    int n;
    vector<edge> edges;
    UnionFind uf;
public:
    Kruskal(int _n) : n(_n), uf(UnionFind(n)) {}
    void addEdge(int _from, int _to, int _cost) {
        edges.emplace_back(_from, _to, _cost);
    }
    int calc() {
        sort(all(edges));
        int res = 0;
        for (auto &e : edges) {
            if (!uf.same(e.from, e.to)) {
                uf.unite(e.from, e.to);
                res += e.cost;
            }
        }
        return res;
    }
};

signed main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);

    int n;
    cin >> n;
    Kruskal kr(n);
    rep(i, n) {
        rep(j, n) {
            int c;
            cin >> c;
            if (c == -1) c = 1e9;
            kr.addEdge(i, j, c);
        }
    }
    cout << kr.calc() << endl;
}

C++: #include<bits/stdc++.h>
using namespace std;
#define int long long
signed main(){
  int n;
  cin>>n;
  int a[n][n];
  for(int i=0;i<n;i++)
    for(int j=0;j<n;j++)
      cin>>a[i][j];
  int ans=0;
  bool used[n];
  memset(used,0,sizeof(used));
  used[0]=1;
  int cost[n];
  for(int i=0;i<n;i++) cost[i]=a[0][i];
  while(1){
    int v=-1;
    for(int i=0;i<n;i++){
      if(used[i]||cost[i]<0) continue;
      //cout<<i<<":"<<cost[i]<<endl;
      if(v<0||cost[i]<cost[v]) v=i;
    }
    if(v<0) break;
    used[v]=1;
    ans+=cost[v];
    //cout<<v<<endl;
    for(int i=0;i<n;i++){
      if(a[v][i]<0) continue;
      if(cost[i]<0||a[v][i]<cost[i]) cost[i]=a[v][i];
    }
  }
  cout<<ans<<endl;
  return 0;
}

C++: #include <iostream>

using namespace std;
int M[100][100]; 
int n; 
static const int WHITE=0; 
static const int GRAY=1; 
static const int BLACK=2; 
static const int INFTY=(1<<21);  
int prim()
{
    int color[100]; 
    int d[100]; 
    int parent[100];
    for(int i=0;i<n;i++)
       {
          d[i]=INFTY; 
          color[i]=WHITE;
       }
    d[0]=0; 
    parent[0]=-1;
    while(1)
    {
        int r=-1;int minv=INFTY;
        for(int i=0;i<n;i++)
        {
            if(color[i]!=BLACK&&d[i]<minv)
            {
                r=i;minv=d[i];
            }
        }
        if(r==-1) break; 
        color[r]=BLACK; 
        for(int i=0;i<n;i++)
        {
            if(color[i]!=BLACK&&M[r][i]!=INFTY) 
            {
                if(d[i]>M[r][i]) 
                {
                    d[i]=M[r][i];
                    parent[i]=r; 
                    color[i]=GRAY;
                }
            }
        }
    }
    int sum=0;
    for(int i=0;i<n;i++)
    {
        if(parent[i]!=-1)
            sum+=M[i][parent[i]];
    }
    return sum;
}
int main()
{
    while(cin>>n)
    {
        for(int i=0;i<n;i++)
            for(int j=0;j<n;j++)
              {
                   cin>>M[i][j];
                   if(M[i][j]==-1) M[i][j]=INFTY;
              }
        cout<<prim()<<endl;
    }
    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;
}