Go: package main

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

const (
	white int = iota
	gray
	black
)
const INFTY int = 2001

var G [][]int
var color []int
var d []int
var p []int
var N int

func prim() int {
	u := 0
	d[u] = 0
	for {
		minCost := INFTY
		for 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 v:=0; v<N; v++ {
			if color[v] != black && G[u][v] != -1 {
				if G[u][v] < d[v] {
					d[v] = G[u][v]
					p[v] = u
					color[v] = gray
				}
			}
		}
	}
	// sum
	sum := 0
	for i:=0; i<N; i++ {
		if p[i] != -1 {
			sum += G[i][p[i]]
		}
	}
	return sum
}




func initVariable(N int) {
	G = make([][]int, N)
	for i:=0; i<N; i++ {
		G[i] = make([]int, N)
	}
	color = make([]int, N)
	d = make([]int, N)
	p = make([]int, N)
	for i:=0; i<N; i++ {
		d[i] = INFTY
		p[i] = -1
	}
}

func nextInt(scanner *bufio.Scanner) int {
	scanner.Scan()
	num, _ := strconv.Atoi(scanner.Text())
	return num
}

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	scanner.Split(bufio.ScanWords)
	N = nextInt(scanner)
	initVariable(N)
	for i:=0; i<N; i++ {
		for j:=0; j<N; j++ {
			G[i][j] = nextInt(scanner)
		}
	}
	fmt.Printf("%d\n", prim())
}

C++: #include <stdio.h>
#include <stdint.h>
#include <iostream>
#include <fstream>
#include <cassert>
#include <climits>

#include <vector>
#include <string>
using namespace std;

#define DEBUG (0)

#if DEBUG
#define DPRINTF(...) do { \
    printf("D(L%d) %s: ", __LINE__, __func__); \
    printf(__VA_ARGS__); \
  } while (false)
#else
#define DPRINTF(...)
#endif // DEBUG


const int kMaxN = 100;
const int kInvalid = INT_MAX;
int g_weightOfVij[kMaxN][kMaxN];
int g_numOfVertices;


struct Edge {
  Edge (int v0, int v1)
    : v0_(v0)
    , v1_(v1)
  { }

  int v0_;
  int v1_;
  int getWeight() { return g_weightOfVij[v0_][v1_]; }
};


struct VisitData {
  VisitData(int num)
    : numOfData_(num)
    , totalWeight_(0)
    {
      memset(visited_, 0, kMaxN);
    }

  bool visited_[kMaxN];
  int numOfData_;
  int totalWeight_;

  bool isAllVisited();

#if DEBUG
  vector<int> path_; // for history of pathes
#endif // DEBUG
};


bool VisitData::isAllVisited()
{
  for (int i = 0; i < numOfData_; i++)
    if (visited_[i] == false)
      return false;

  return true;
}


void printAllWeights()
{
  DPRINTF("Number of vertices %d\n", g_numOfVertices);

  for (int i = 0; i < g_numOfVertices; i++)
  {
    for (int j = 0; j < g_numOfVertices; j++)
    {
      if (j == 0)  DPRINTF("");
      printf(" %d", g_weightOfVij[i][j]);
    }
    cout << endl;
  }
}


void printAllPath()
{
  DPRINTF("Number of vertices %d\n", g_numOfVertices);
  DPRINTF("$src -> $dst ($weight)\n");

  for (int i = 0; i < g_numOfVertices; i++)
  {
    for (int j = 0; j < g_numOfVertices; j++)
    {
      if (g_weightOfVij[i][j] > 0)
        DPRINTF("%d -> %d (%d)\n", i, j, g_weightOfVij[i][j]);
    }
  }
}


void getConnectedVertices(int pos, vector<int> &vertices)
{
  for (int i = 0; i < g_numOfVertices; i++)
    if (g_weightOfVij[pos][i] > 0)
      vertices.push_back(i);

#if 0
  DPRINTF("connected points from %d :", pos);
  for (int i = 0; i < vertices.size(); i++)
    printf(" %d", vertices[i]);

  cout << endl;
#endif
}


/* find unvisited vertices that are connected from the vertices already visited */
void getNextCandidates(const bool visited[kMaxN], vector<Edge> &candidates)
{
#if 0
  DPRINTF("visited:\n");
  for (int i = 0; i < g_numOfVertices; i++)
    DPRINTF("  [%d] = %d\n", i, visited[i]);
#endif

  for (int startV = 0; startV < g_numOfVertices; startV++)
  {
    if (visited[startV] == true)
    {
      // Get all connected vertices
      vector<int> vertices;
      getConnectedVertices(startV, vertices);

      // Remove vertices that are already visited
      for (int nextV : vertices)
      {
        if (visited[nextV] == false)
          candidates.push_back(Edge(startV, nextV));
      }
    }
  }

#if 0
  DPRINTF("Next candidates:\n");
  for (int i = 0; i < candidates.size(); i++)
    printf("  %d -> %d\n", candidates[i].v0_, candidates[i].v1_);
#endif
}


//int findSpanningTree(int vertex, VisitData info /* Not pointer nor reference */)
int findSpanningTree(int vertex, VisitData &info)
{

  assert(info.visited_[vertex] == false);
  info.visited_[vertex] = true;

#if DEBUG
  info.path_.push_back(vertex);
#endif

#if 1 & DEBUG
  DPRINTF("Path:");
  for (int i = 0; i < info.path_.size(); i++)
    printf(" %d", info.path_[i]);
  cout << endl;
#endif

  if (info.isAllVisited())
  {
    DPRINTF("all vertices visited. total weight %d\n", info.totalWeight_);
    return info.totalWeight_;
  }

  vector<Edge> nextPosCandidates;
  getNextCandidates(info.visited_, nextPosCandidates);

  int minWeight = kInvalid;

  for (int i = 0; i < nextPosCandidates.size(); i++)
  {
    const int startV = nextPosCandidates[i].v0_;
    const int nextV = nextPosCandidates[i].v1_;

    VisitData nextInfo = info;
    nextInfo.totalWeight_ += g_weightOfVij[startV][nextV];

    const int weight = findSpanningTree(nextV, nextInfo);

    if (weight != kInvalid && weight < minWeight)
      minWeight = weight;
  }

  DPRINTF("minWeight %d\n", minWeight);
#if 1 & DEBUG
  DPRINTF("  Path:");
  for (int i = 0; i < info.path_.size(); i++)
    printf(" %d", info.path_[i]);
  cout << endl;
#endif

  return minWeight;
}


int main()
{
  cin >> g_numOfVertices; cin.ignore();

  DPRINTF("Number of vertices %d\n", g_numOfVertices);

  for (int i = 0; i < g_numOfVertices; i++)
  {
    for (int j = 0; j < g_numOfVertices; j++)
      cin >> g_weightOfVij[i][j];

    cin.ignore();
  }

#if DEBUG
  DPRINTF("Number of vertices %d\n", g_numOfVertices);
  printAllWeights();
  //printAllPath();
#endif // DEBUG

  const int startPos = 0;
  VisitData info(g_numOfVertices);

  cout << findSpanningTree(startPos, info) << endl;

  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }

using Lint=long long;

class UnionFind
{
 public:
    vector<int> par,sz;
    UnionFind(int N):par(N),sz(N){
        for(int i=0;i<N;i++){
            par[i]=i; sz[i]=1;
        }
    }
    int find(int x){
        if(par[x]==x) return x;
        return par[x]=find(par[x]);
    }
    void unite(int x,int y){
        x=find(x); y=find(y);
        if(x==y) return;
        if(sz[x]<sz[y]) swap(x,y);
        par[y]=x;
        sz[x]+=sz[y];
    }
    bool same(int x,int y){ return find(x)==find(y); }
    int size(int x){ return sz[find(x)]; }
    bool root(int x){ return par[x]==x; }
};

struct edge{
    int u,v,cost;
    bool operator<(const edge &e){
        return cost<e.cost;
    }
};

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);

    int N; cin>>N;
    vector<edge> es;
    for(int i=0;i<N;i++){
        for(int j=0;j<N;j++){
            int c; cin>>c;
            if(i<j and c!=-1) es.push_back({i,j,c});
        }
    }
    sort(es.begin(),es.end());

    int ans=0;
    UnionFind tree(N);
    for(edge e:es){
        if(!tree.same(e.u,e.v)){
            ans+=e.cost;
            tree.unite(e.u,e.v);
        }
    }
    cout<<ans<<endl;
    return 0;
}