Go: package main

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

const (
	WHITE int = iota
	GRAY
	BLACK
)

type graph struct {
	G [][]int
	C []int
	D []int
	P []int
}

var INFTY = 1 << 32

func constructGraph() *graph {
	sc := bufio.NewScanner(os.Stdin)
	sc.Scan()
	n, _ := strconv.Atoi(sc.Text())

	g := make([][]int, n)
	c := make([]int, n)
	d := make([]int, n)
	p := make([]int, n)

	for i := 0; i < n; i++ {
		sc.Scan()
		fields := strings.Fields(sc.Text())
		g[i] = toIntSlice(fields)
	}

	return &graph{g, c, d, p}
}

func (g *graph) initMst() {
	for i := 0; i < len(g.G); i++ {
		g.C[i] = WHITE
		g.D[i] = INFTY
	}
	g.D[0] = 0
	g.P[0] = -1
}

func (g *graph) primMst() int {
	g.initMst()

	var u int
	for {
		mincost := INFTY
		for i := 0; i < len(g.G); i++ {
			if g.C[i] != BLACK && g.D[i] < mincost {
				mincost = g.D[i]
				u = i
			}
		}

		if mincost == INFTY {
			break
		}

		g.C[u] = BLACK

		for i := 0; i < len(g.G); i++ {
			if g.C[i] != BLACK && g.existEdge(u, i) {
				if g.G[u][i] < g.D[i] {
					g.D[i] = g.G[u][i]
					g.P[i] = u
				}
			}
		}
	}

	totalCost := 0
	for _, cost := range g.D {
		totalCost += cost
	}
	return totalCost
}

func (g *graph) existEdge(u, v int) bool {
	return g.G[u][v] != INFTY && g.G[v][u] != INFTY
}

func toIntSlice(s []string) []int {
	a := make([]int, len(s))
	for i, v := range s {
		num, _ := strconv.Atoi(v)
		if num == -1 {
			num = INFTY
		}
		a[i] = num
	}
	return a
}

func main() {
	g := constructGraph()
	fmt.Println(g.primMst())
}

C++: #define _CRT_SECURE_NO_WARNINGS
#include <cstdio>
#include <cstdlib>
#include <string>
#include <iostream>
#include <climits>
#include <list>
#include <queue>

using namespace std;

static const int INFTY = 2001;

int main() {
	int Dist[101];
	int AdjacenceyList[100][100];
	int AdjacenceyWeightList[100][100];
	int Degree[100];
	int i, j;
	int N;
	int W;
	int Deg;
	int MinTotalWeight = 0;
	int u, v;
	int PrevSecondSmallestVertex;
	int SecondSmallestVertex;
	int MinVertex;
	bool inT[100];

	cin >> N;

	for (i = 0; i < N; i++) {
		inT[i] = false;
		Dist[i] = INFTY;
		Deg = -1;
		for (j = 0; j < N; j++) {
			cin >> W;
			if(W != -1){
				AdjacenceyList[i][++Deg] = j;
				AdjacenceyWeightList[i][Deg] = W;
			}
		}
		Degree[i] = Deg + 1;
	}
	Dist[100] = INFTY;

	u = 0;
	inT[u] = true;
	Dist[u] = 0;
	PrevSecondSmallestVertex = 100;
	for (i = 1; i <= N - 1; i++) {
		MinVertex = 100;
		for (j = 0; j < Degree[u]; j++) {
			v = AdjacenceyList[u][j];
			if (inT[v] == false) {
				W = AdjacenceyWeightList[u][j];
				if (W < Dist[v]) {
					Dist[v] = W;
				}
				if (Dist[v] < Dist[MinVertex]) {
					SecondSmallestVertex = MinVertex;
					MinVertex = v;
				}
				else if (Dist[v] < Dist[SecondSmallestVertex]) {
					SecondSmallestVertex = v;
				}
			}
		}

		////DEBUG
		//cout << "MinVerTex = " << MinVertex << " SecondSmallestVertex = " << SecondSmallestVertex << endl;

		if (Dist[PrevSecondSmallestVertex] < Dist[MinVertex]) {
			SecondSmallestVertex = MinVertex;
			MinVertex = PrevSecondSmallestVertex;
			PrevSecondSmallestVertex = SecondSmallestVertex;
		}
		else if (Dist[SecondSmallestVertex] < Dist[PrevSecondSmallestVertex]) {
			PrevSecondSmallestVertex = SecondSmallestVertex;
		}
		inT[MinVertex] = true;
		u = MinVertex;
		MinTotalWeight += Dist[MinVertex];

		////DEBUG
		//cout << "MinVertex = " << MinVertex << " PrevSecondSmallestVertex = " << PrevSecondSmallestVertex << endl;
		//cout << "inT: ";
		//for (int k = 0; k < N; k++) {
		//	cout << inT[k] << " ";
		//}
		//cout << endl;
		//printf("MinTotalWeight = %d\n", MinTotalWeight);
	}
	printf("%d\n", MinTotalWeight);

	return 0;
}

C++: #include <stdio.h>
using namespace std;
static const int MAX = 100;
static const int INFTY = (1<<21);
static const int WHITE = 0;
static const int GRAY = 0;
static const int BLACK = 0;

int n, M[MAX][MAX];

int prim() {
    int d[n], p[n];
    int color[n];
    
    for(int i=0; i<n; i++) {
        color[i] = WHITE;
        d[i] = INFTY;
        p[i] = -1;
    }
    d[0] = 0;
    p[0] = -1;
    
    while(1) {
        int u;
        int mincost = INFTY;
        for (int i=0; i<n; i++) {
            if (color[i] != BLACK && d[i] < mincost) {
                mincost = d[i];
                u = i;
            }
        }
        
        if (mincost == INFTY) break;
        
        color[u] = BLACK;
        
        for (int v=0; v<=n; v++) {
            if (color[v] != BLACK && M[u][v] != INFTY) {
                if(M[u][v] < d[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 tmp;
            scanf("%d", &tmp);
            M[i][j] = (tmp == -1) ? INFTY : tmp;
        }
    }
    
    int sum = prim();
    printf("%d\n", sum);
    
    return 0;
}

C++: #include <cstdio>
#include <map>
#include <algorithm>

#define rep(i, n) for(int i = 0; i < (n); ++i)

using namespace std;

int n;
int g[100][100];

vector<pair<int, int> > computeSpanningTree(std::vector<pair<int, pair<int, int> > >& edges){
  class UnionFind{
  public:
    UnionFind(int n){
      for(int i = 0; i < n; ++i){
        parent.push_back(i);
      }
      rank.resize(n, 0);
    }

    bool same(int a, int b){
      return find(a) == find(b);
    }

    void unite(int a, int b){
      int p = find(a), q = find(b);

      // assume that p != q

      if(rank[p] > rank[q]){
        parent[q] = p;
      }
      else{
        parent[p] = q;
        
        if(rank[p] == rank[q]){
          ++rank[q];
        }
      }
    }

  private:
    int find(int a){
      return a == parent[a] ? a : parent[a] = find(parent[a]);
    }

  private:
    vector<int> parent, rank;
  };

  class Indexer{
  public:
    int add(int p){
      if(indexer.find(p) == indexer.end()){
        int new_id = indexer.size();
        indexer[p] = new_id;
      }
      return indexer[p];
    }

  private:
    std::map<int, int> indexer;
  };

  sort(edges.begin(), edges.end());

  Indexer id;
  UnionFind uf(edges.size() * 2);

  vector<pair<int, int> > result;
  for(int i = 0; i < edges.size(); ++i){
    int from = edges[i].second.first;
    int to = edges[i].second.second;

    int a = id.add(from);
    int b = id.add(to);

    if(!uf.same(a, b)){
      result.push_back(make_pair(from, to));
      uf.unite(a, b);
    }
  }

  return result;
}

int main(){
	scanf("%d", &n);
	rep(i, n){
		rep(j, n){
			scanf("%d", &g[i][j]);
		}
	}

	std::vector<pair<int, pair<int, int> > > gr;
	rep(i, n){
		for(int j = i; j < n; ++j){
			if(g[i][j] != -1){
				gr.push_back(make_pair(g[i][j], make_pair(i, j)));
			}
		}
	}
	vector<pair<int, int> > edge = computeSpanningTree(gr);
	int s = 0;
	rep(i, edge.size()){
		s += g[edge[i].first][edge[i].second];
	}
	printf("%d\n", s);
	return 0;
}