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++: /* minimum_spanning_tree_prim.cpp
    プリム法で最小全域木とそのコストを求める（隣接行列 向き）
    計算量: O(|E|log|V|)

    verified: AOJ ALDS1_12_A Minimum Spanning Tree
        http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ALDS1_12_A&lang=jp
 */

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

vector<vector<int>> G;  // 隣接行列での表現

/* Prim : プリム法で minimum spanning tree を求める構造体
    入力: 隣接行列でのグラフ G
    最小全域木の重みの総和: sum
    計算量: O(|V|^2)
*/
struct Prim {
    const int INF = 1e9;
    int sum;              // MSTの重みの総和
    int n;                // 頂点数
    vector<int> mincost;  // 既に確定した頂点からの最小コスト（確定済みから行けない時は INF）
    vector<bool> used;    // 既に確定したかどうか
    Prim(vector<vector<int>> const& Graph) {
        init(Graph);
    }
    void init(vector<vector<int>> const& Graph) {
        n = (int)Graph.size();
        mincost.assign(n, INF);
        used.assign(n, false);
        sum = 0;
        mincost[0] = 0;  // 始め頂点0からスタートさせる
        while (true) {
            int v = -1;
            for (int u = 0; u < n; u++) {  // コストが最小で行ける頂点 v を探す
                if (!used[u] && (v == -1 || mincost[u] < mincost[v])) v = u;
            }
            if (v == -1) break;  // MST ができたので終了
            used[v] = true;
            sum += mincost[v];
            for (int u = 0; u < n; u++) {  // 確定した頂点から行ける頂点について、最小コストを更新
                if (Graph[v][u] != -1) mincost[u] = min(mincost[u], Graph[v][u]);
            }
        }
    }
};

int main() {
    int n;
    cin >> n;
    G.assign(n, vector<int>(n));
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            cin >> G[i][j];
        }
    }
    Prim prim(G);
    cout << prim.sum << endl;
    return 0;
}

C++: #include <iostream>

using namespace std;

#define INT_MIN     (-2147483647 - 1) /* minimum (signed) int value */
#define INT_MAX       2147483647    /* maximum (signed) int value */
static const int MAX = 100;
static const int WHITE = 0;
static const int GRAY = 1;
static const int BLACK = 2;

int n, m[MAX][MAX];

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

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

	d[0] = 0;

	while (1)
	{
		minv = INT_MAX;
		u = -1;
		for (int i = 0; i < n; ++i)
		{
			if (d[i] < minv && color[i] != BLACK)
			{
				u = i;
				minv = d[i];
			}
		}

		if (-1 == u) 
			break;
		color[u] = BLACK;

		for (int i = 0; i < n; ++i)
		{
			if (m[u][i] < d[i] && color[i] != BLACK)
			{
				d[i] = m[u][i];
				p[i] = u;
				color[i] = GRAY;
			}
		}
	}

	int sum = 0;
	for (int i = 0; i < n; ++i)
	{
		if (p[i] != -1) 
			sum += m[p[i]][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) ? INT_MAX : e;
		}
	}
	cout << prim() << endl;
}

C++: #include <bits/stdc++.h>
#define rep(i,a) for(i=0;i<a;i++)
using namespace std;
static const int W 0
static const int G 1
static const int B 2

void prim(int n, int *total, vector<vector<int> > &M) {
    int i,u,min;
    vector<int> d(n);
    vector<int> p(n);
    vector<int> color(n);

    rep(i,n){
        d[i] = INT_MAX;
        p[i] = -1;
        color[i] = INT_MAX;
    }
    d[0] = 0;
    while(true){
        min = INT_MAX;
        u = -1;
        rep(i,n){
            if(min > d[i] && color[i] != B) {
                u = i;
                min = d[i];
            }
        }
        if(u==-1) break;
        color[u] = B;
        rep(i,n){
            if(color[i] != B && M[u][i])
                if(d[i] > M[u][i]) {
                    d[i] = M[u][i];
                    p[i] = u;
                    color[u] = G;
                }
        }
    }
    rep(i,n){
        if(p[i] != -1) total += M[i][p[i]]
    }
}

int main(void) {
    int i,j,n,total = 0;
    vector<vector<int> > M;
    cin >> n;
    M.resize(n);

    rep(i,n){
        M[i].resize(n);
    }
    rep(i,n){
        rep(j,n){
            cin >> M[i][j];
            if(M[i][j] != -1) M[i][j] = INT_MAX;

        }
    }
    prim(n,&total,M);
    cout << total;
    return 0;
}

C++: #include <stdio.h>
#define INFTY 1000000
#define M -1
#define WHITE 0
#define GRAY  1
#define BLACK 2
#define N 100
int prim(void);
int n,A[N][N];

int main()
{
  int i,j,k;

  scanf("%d",&n);
  for(i=0;i<n;i++)
    {
      for(j=0;j<n;j++)
        {
          scanf("%d",&k);
          if(k==M) A[i][j]=INFTY;
        }
    }

  printf("%d\n",prim());
  return 0;
}

int prim()
{
  int d[N],p[N],color[N];
  int i,v,u,mincost,sum=0;

  for(i=0;i<n;i++)
    {
      d[i]=INFTY;
      p[i]=M;
      color[i]=WHITE;
    }
  d[0]=0;
while(1)
    {
      mincost=INFTY;
      u=M;
      for(i=0;i<n;i++)
        {
          if(color[i]!=BLACK&&d[i]<mincost)
            {
              u=i;
              mincost=d[i];
            }
        }
      if(u==M) break;
      color[u]=BLACK;
      for(v=0;v<n;v++)
        {
          if(color[v]!=BLACK&&A[u][v]!=INFTY)
            {
              if(d[v]>A[u][v])
                {
                  d[v]=A[u][v];
                  p[v]=u;
                  color[v]=GRAY;
                }
            }
        }
    }
  for(i=0;i<n;i++)
    {
      if(p[i]!=M) sum+=A[i][p[i]];
    }
  return sum;