Go: package main

import (
	"sort"
	"bufio"
	"os"
	"fmt"
	"strconv"
)

// Define disjoint-set forests

type vertexNo int

type vertex struct {
	number vertexNo
	rank int
	parent *vertex
}

func makeSet(number vertexNo) vertex {
	return vertex {
		number: number,
		rank: 0,
		parent: nil,
	}
}

func union(x, y *vertex) *vertex {
	return link(findSet(x), findSet(y))
}

func link(x, y *vertex) *vertex {
	if x.rank > y.rank {
		y.parent = x
		return x
	} else {
		x.parent = y
		if x.rank == y.rank {
			y.rank += 1
		}
		return y
	}
}

func findSet(v *vertex) *vertex {
	if v.parent == nil {
		return v
	}
	v.parent = findSet(v.parent)
	return v.parent
}

// Function to calculate minimum spanning tree

type matrix [][]int

type edge struct {
	one vertexNo
	two vertexNo
	weight int
}

const(
	MAX_VERTICES = 100
)

func newEdgesFromMatrix(matrix matrix) []edge {
	edges := make([]edge, 0, MAX_VERTICES * MAX_VERTICES)

	for i := 0; i < len(matrix); i++ {
		row := matrix[i]
		for j := i; j < len(row); j++ {
			if row[j] >= 0 {
				edge := edge{one: vertexNo(i+1), two: vertexNo(j+1), weight: row[j]}
				edges = append(edges, edge)
			}
		}
	}

	return edges
}

func solve(matrix matrix) int {
	nVertices := len(matrix)

	// initialize vertices
	vertices := make([]vertex, nVertices)
	for i := 0; i < nVertices; i++ {
		vertices[i] = makeSet(vertexNo(i+1))
	}

	// prepare edges sorted in the ascending order of weight
	edges := newEdgesFromMatrix(matrix)
	sort.Slice(edges, func(i, j int) bool {
		return edges[i].weight < edges[j].weight
	})

	unionCount := 0
	weightSum := 0
	for unionCount < nVertices-1 {
		for i := 0; i < len(edges); i++ {
			edge := edges[i]
			v1 := &vertices[edge.one-1]
			v2 := &vertices[edge.two-1]
			rep1 := findSet(v1)
			rep2 := findSet(v2)
			if rep1.number == rep2.number {
				continue
			}
			union(rep1, rep2)
			edges = append(edges[0:i], edges[i+1:]...)
			weightSum += edge.weight
			unionCount++
			break
		}
	}

	return weightSum
}

// main

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)

	n, err := nextInt(sc)
	if err != nil {
		panic(err)
	}

	matrix := parseMatrix(n, sc)

	edges := newEdgesFromMatrix(matrix)
	sort.Slice(edges, func(i, j int) bool {
		return edges[i].weight < edges[j].weight
	})

	fmt.Println(solve(matrix))
}

func parseMatrix(n int, sc *bufio.Scanner) [][]int {
	matrix := make([][]int, n)
	for i := 0; i < n; i++ {
		matrix[i] = make([]int, n)
		for j := 0; j < n; j++ {
			x, err := nextInt(sc)
			if err != nil {
				panic(err)
			}
			matrix[i][j] = x
		}
	}
	return matrix
}

func nextText(sc *bufio.Scanner) string {
	sc.Scan()
	return sc.Text()
}

func nextInt(sc *bufio.Scanner) (int, error) {
	return strconv.Atoi(nextText(sc))
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
#define REP(i, n) for (long long i = 0; i < (n); i++)
struct UnionFind {
    vector<int> parent, size;
    UnionFind(int n) {
        parent.resize(n, -1);
        size.resize(n, 1);
    }
    void unite(int x, int y) {
        if (same(x, y)) {
            return;
        }
        x = root(x);
        y = root(y);
        if (size[x] <= size[y]) {
            swap(x, y);
        }
        parent[y] = x;
        size[x] += size[y];
    }
    bool same(int x, int y) { return (root(x) == root(y)); }
    int root(int x) {
        while (parent[x] != -1) {
            x = parent[x];
        }
        return x;
    }
    int getsize(int x) { return size[root(x)]; }
};
vector<vector<P>> mst(vector<vector<P>> graph) {
    vector<pair<ll, P>> as;
    REP(i, graph.size()) {
        REP(j, graph[i].size()) { as.push_back({graph[i][j].second, {i, graph[i][j].first}}); }
    }
    sort(as.begin(), as.end());
    UnionFind uf(graph.size());
    vector<vector<P>> ans(graph.size());
    REP(i, as.size()) {
        if (!uf.same(as[i].second.first, as[i].second.second)) {
            ans[as[i].second.first].push_back({as[i].second.second, as[i].first});
            ans[as[i].second.second].push_back({as[i].second.first, as[i].first});
            uf.unite(as[i].second.second, as[i].second.first);
        }
    }
    return ans;
}
signed main() {
    int v;
    cin >> v;
    vector<vector<P>> graph(v);
    vector<vector<P>> ans(v);
    REP(i, v) {
        REP(j, v) {
            int a;
            cin >> a;
            if (a != -1) {
                graph[i].push_back({j, a});
            }
        }
    }
    ans = mst(graph);
    ll sum = 0;
    REP(i, v) {
        REP(j, ans[i].size()) { sum += ans[i][j].second; }
    }
    cout << sum / 2 << endl;
}

C++: #include<bits/stdc++.h>
using namespace std;
  static const int MAX=100;
  static const int INF=(1<<21);
  static const int WHI=0;
  static const int GRA=1;
  static const int BLA=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]=INF;
      p[i]=-1;
      color[i]=WHI;
    }
    
    d[0]=0;
    while(1){
      minv=INF;
      u=-1;
      for(int i=0;i<n;i++){
	if(minv>d[i]&&color[i]!=BLA){
	  u=i;
	}
      }

      if(u==-1){
	break;
      }
      color[u]=BLA;

      for(int v=0;v<n;v++){
	if(color[v]!=BLA&&M[u][v]!=INF){
	  if(M[u][v]<d[v]){
	    d[v]=M[u][v];
	    p[v]=u;
	    color[v]=GRA;
	  }

	}
      }
    }

    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;
	if(e==-1){
	  M[i][j]=INF;
	}else{
	  M[i][j]=e;
	}
      }
    }
    
    cout<<prim()<<endl;
    return 0;
  }

C++: #include"bits/stdc++.h"
using namespace std;

#define NIL -1
#define MAX (1 << 24)

struct Node {
	int dis = MAX;
	bool flag = false;
	vector<int> d;
	vector<int> id;
};

void dfs(vector<Node>& u, int id)
{
	u[0].dis = 0;

	while (1)
	{
		bool fin = true;
		int mini = MAX;
		int n;
		for (int i = 0; i < u.size(); i++)
		{
			if (!u[i].flag && u[i].dis < mini) {
				mini = u[i].dis;
				n = i;
				fin = false;
			}
		}

		if (fin) return;
		u[n].flag = true;

		for (int i = 0; i < u[n].id.size(); i++)
		{
			if (u[u[n].id[i]].dis > u[n].d[i] && !u[u[n].id[i]].flag) {
				u[u[n].id[i]].dis = u[n].d[i];
			}
		}
	}
}

int main(void)
{
	int i, N, m, q = 0;
	cin >> N;
	vector<Node> u(N);
	for (i = 0; i < N; i++)
	{
		for (int j = 0; j < N; j++)
		{
			cin >> m;
			if (m == NIL) continue;
			u[i].d.push_back(m);
			u[i].id.push_back(j);
		}
	}

	
	dfs(u, 0);

	for (i = 0; i < N; i++) q += u[i].dis;

	cout << q << endl;

	return 0;
}