Go: package main

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

const (
	MAX   = 100
	INFTY = 1 << 21
	NIL   = -1
)

const (
	WHITE = iota
	GRAY
	BLACK
)

type Graph [MAX][MAX]int

func (g *Graph) prim(n int) int {
	var (
		u, minv     = 0, 0
		d, p, color [MAX]int
	)

	for i := 0; i < n; i++ {
		d[i] = INFTY
		p[i] = -1
		color[i] = WHITE
	}
	d[0] = 0

	for {
		minv = INFTY
		u = -1
		for 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 v := 0; v < n; v++ {
			if color[v] != BLACK && g[u][v] != INFTY {
				if d[v] > g[u][v] {
					d[v] = g[u][v]
					p[v] = u
					color[v] = GRAY
				}
			}
		}
	}

	sum := 0
	for i := 0; i < n; i++ {
		if p[i] != -1 {
			sum += g[i][p[i]]
		}
	}

	return sum
}

func answer(reader io.Reader) string {
	sc := bufio.NewScanner(reader)
	sc.Split(bufio.ScanWords)

	sc.Scan()
	n, _ := strconv.Atoi(sc.Text())

	var g Graph
	for i := 0; i < n; i++ {
		for j := 0; j < n; j++ {
			sc.Scan()
			e, _ := strconv.Atoi(sc.Text())
			if e == -1 {
				g[i][j] = INFTY
				continue
			}
			g[i][j] = e
		}
	}

	return fmt.Sprint(g.prim(n))
}

func main() {
	fmt.Println(answer(os.Stdin))
}

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 H;
int A[101];
int NodeNum[101];
int IndexInA[100];


int parent(int i) {
	return i / 2;
}

int left(int i) {
	return 2 * i;
}

int right(int i) {
	return 2 * i + 1;
}

void minHeapify(int i) {
	int l = left(i);
	int r = right(i);
	int smallest = i;
	if (l <= H && A[l] < A[i]) {
		smallest = l;
	}
	if (r <= H && A[r] < A[smallest]) {
		smallest = r;
	}
	if (smallest != i) {
		int temp = A[i];
		A[i] = A[smallest];
		A[smallest] = temp;
		temp = NodeNum[i];
		NodeNum[i] = NodeNum[smallest];
		NodeNum[smallest] = temp;
		IndexInA[NodeNum[i]] = i;
		IndexInA[NodeNum[smallest]] = smallest;

		minHeapify(smallest);
	}
}

void insert(int d, int nodeNum) {
	int i = ++H;
	A[i] = d;
	NodeNum[i] = nodeNum;
	IndexInA[nodeNum] = i;
	int p = parent(i);
	while (i > 1 && A[p] > A[i]) {
		int temp = A[p];
		A[p] = A[i];
		A[i] = temp;
		temp = NodeNum[p];
		NodeNum[p] = NodeNum[i];
		NodeNum[i] = temp;
		IndexInA[NodeNum[p]] = p;
		IndexInA[NodeNum[i]] = i;
		i = p;
		p = parent(i);
	}
}

int heapExtractMinNodeNum() {
	int minNodeNum = NodeNum[1];
	A[1] = A[H];
	NodeNum[1] = NodeNum[H];
	IndexInA[NodeNum[1]] = 1;
	H--;
	minHeapify(1);

	return minNodeNum;
}

void decrese(int i, int value) {
	A[i] = value;
	int p = parent(i);
	while (i > 1 && A[p] > A[i]) {
		int temp = A[p];
		A[p] = A[i];
		A[i] = temp;
		temp = NodeNum[p];
		NodeNum[p] = NodeNum[i];
		NodeNum[i] = temp;
		IndexInA[NodeNum[p]] = p;
		IndexInA[NodeNum[i]] = i;
		i = p;
		p = parent(i);
	}
}

int main() {
	int Dist[10];
	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;
	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;
	}



	H = 0;
	u = 0;
	inT[u] = true;
	Dist[u] = 0;
	for (i = 1; i <= N - 1; i++) {
		for (j = 0; j < Degree[u]; j++) {
			v = AdjacenceyList[u][j];
			if (inT[v] == false) {
				W = AdjacenceyWeightList[u][j];
				if (Dist[v] == INFTY) {
					Dist[v] = W;
					insert(W, v);
				}
				else if (W < Dist[v]) {
					Dist[v] = W;
					decrese(IndexInA[v], W);
				}
			}
		}

		int minNodeNum = heapExtractMinNodeNum();
		inT[minNodeNum] = true;
		MinTotalWeight += Dist[minNodeNum];
		u = minNodeNum;

	}

	printf("%d\n", MinTotalWeight);

	return 0;
}

C++: #include <iostream>

using namespace std;

const int MAX = 100;
const int INFTY = (1 << 21);

class MST
{
    int num, G[MAX][MAX], status[MAX];

public:
    MST()
    {
        int value;
        cin >> num;
        for (int i = 0; i < num; i++)
        {
            for (int j = 0; j < num; j++)
            {
                cin >> value;
                G[i][j] = value == -1 ? INFTY : value;
            }
        }
        for (int i = 0; i < num; i++)
        {
            status[i] = 0;
        }
    }

    int prim(int node)
    {
        int min_index1, min_index2, sum = 0;
        status[node] = 1;
        for (int j = 0; j < num-1; j++)
        {
            min_index1 = min_index2 = 0;
            for (int n = 0; n < num; n++)
            {
                if (status[n] != 0)
                {
                    for (int i = 1; i < num; i++)
                    {
                        if (status[i] == 0 && G[min_index1][min_index2] > G[n][i])
                        {
                            min_index1 = n;
                            min_index2 = i;
                        }
                    }
                }
            }
            status[min_index2] = 1;
            sum += G[min_index1][min_index2];
        }
        return sum;
    }
};

int main()
{
    MST mst;
    cout << mst.prim(0) << endl;
}

C++: #include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdint.h>
#include <tuple>
#include <utility>
#include <vector>

using namespace std;

typedef long long ll;
typedef unsigned int uint;
typedef unsigned long long ull;
static const double EPS = 1e-12;
static const double PI = acos(-1.0);

#define FOR(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define REP(i, n) for (int i = 0; i < (int)(n); i++)
#define ALL(a) (a).begin(), (a).end()
#define DEBUG(x) cout << #x << ": " << x << endl

static const ll INF = std::numeric_limits<ll>::max() / 3;
static const ll MAX_V = 100;  // TODO

ll cost[MAX_V][MAX_V];  // TODO
ll mincost[MAX_V];
bool used[MAX_V];
ll V;  // TODO

typedef pair<ll, ll> Node;

int prim() {
  priority_queue<Node, vector<Node>, greater<Node>> q;
  q.push(Node{0, 0});
  ll res = 0;
  while (!q.empty()) {
    Node node = q.top();
    q.pop();
    if (used[node.second]) {
      continue;
    }
    used[node.second] = true;
    res += node.first;
    for (ll u = 0; u < V; u++) {
      if (cost[node.second][u] != INF) {
        q.push(Node{cost[node.second][u], u});
      }
    }
  }
  return res;
}

void solve(long long n, vector<vector<long long>> a) {
  V = n;
  REP(i, n) {
    REP(j, n) {
      if (a[i][j] == -1) {
        cost[i][j] = INF;
      } else {
        cost[i][j] = a[i][j];
      }
    }
  }
  std::cout << prim() << std::endl;
}

int main() {
  long long n;
  scanf("%lld", &n);
  vector<vector<long long>> a(n, vector<ll>(n));
  REP(i, n) {
    REP(j, n) {
      scanf("%lld", &a[i][j]);
    }
  }
  solve(n, std::move(a));
  return 0;
}