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>
#define REP(i,n) for(long long i=0;i<n;++i)
#define FOR(i,a,b) for(long long i=a;i<b;++i)
using namespace std;
const int MAX=100,INF=1<<30,WHI=0,GRA=1,BLA=2;
int n,m[MAX][MAX];
int prim(){
    int u,minv;
    int d[MAX],p[MAX],color[MAX];
    fill(d,d+n+1,INF);
    fill(p,p+n+1,-1);
    fill(color,color+n+1,WHI);
    d[0]=0;
    while(1){
        minv=INF;
        u=-1;
        REP(i,n){
            if(minv>d[i]&&color[i]!=BLA){
                u=i;
                minv=d[i];
            }
        }
        if(u==-1) break;
        color[u]=BLA;
        REP(v,n){
            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;
    REP(i,n){
        if(p[i]!=-1) sum+=m[i][p[i]];
    }
    return sum;
}
int main(){
    scanf("%d",&n);
    REP(i,n){
        REP(j,n){
            int e;
            scanf("%d",&e);
            m[i][j]=(e==-1)?INF:e;
        }
    }
    printf("%d\n",prim());
    return 0;
}

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

typedef long long ll;


class Union_Find {
private:
    std::vector<int> par;
    std::vector<int> rank;

public:
    Union_Find(int n) { // ?????????: O(n)
	par = rank = std::vector<int>(n);
	for(int i=0; i<n; i++) {
	    par[i]  = i;
	    rank[0] = 0;
	}
    }

    int root(int x) { // ??¨??????????±???????: O(?±(n)) 
	if(par[x] == x) return x;
	return par[x] = root(par[x]); // ??????????????????????????????????????????
    }

    void unite(int x, int y) { // x,y????????????????????????: O(?±(n)) 
	x = root(x); y = root(y);
	if( x == y ) return;

	if( rank[x] < rank[y] ) par[x] = y;
	else {
	    par[y] = x;
	    if( rank[x] == rank[y] ) rank[x]++;
	}
    }

    bool same(int x, int y) { //x,y??????????????????????????????????????????: O(?±(n)) 
	return root(x) == root(y);
    }
};

struct Edge {
    ll s,t;
    ll w;
    bool operator < (const Edge& tgt) const {
	return w > tgt.w;
    }
};

ll Kruskal(priority_queue<Edge>& edges, const int V) {

    ll res = 0;
    
    Union_Find uf(V);
    while( !edges.empty() ) {
	Edge e = edges.top(); edges.pop();
	if( !uf.same(e.s, e.t) ) {
	    uf.unite(e.s, e.t);
	    res += e.w;
	}
    }
    return res;
}


int main () {

    ll V;
    cin >> V;

    priority_queue<Edge> edges;
    
    for(int i=0; i<V; i++) {
	for(int j=0; j<V; j++) {
	    ll d;
	    cin >> d;
	    if( d != -1 )
		edges.push( Edge{i,j,d} );
	}	
    }

    ll res = Kruskal(edges,V);
    cout << res << endl;    
    return 0;
}

C++: #include <bits/stdc++.h>
#define REP(i,n) for (int i = 0; i <(n); ++i)
#define REP2(i,x,n) for (int i = x; i <(n); ++i)
#define ALL(v) v.begin(), v.end()
#define RALL(v) v.rbegin(), v.rend()
using namespace std;
using ll = long long;
using P = pair<int,int>;
static const double PI = acos(-1);
static const int INF = 1e9+7;

/*
  最小全域木

  重み付きグラフの最小全域木の辺の重みの総和を計算する．
  最小全域木とは？：グラフで，すべての頂点に到達できるものの中で，　辺の重みが最小のもの
  プリムのアルゴリズム

*/

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];
  //d: MSTに属する頂点とそれ以外の頂点の中で，重みが最小の辺の重み
  //p: p[v]  MSTにおける頂点vの親
  //color 訪問状態

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

  d[0] = 0;

  while( true ){
    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;

  //入力
  REP(i,n)REP(j,n){
    int e;
    cin >> e;
    if(e == -1) M[i][j] = INFTY;
    else M[i][j] = e;
  }

  cout << prim() << endl;

  return 0;
}