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 <iostream> // cout, endl, cin
#include <string> // string, to_string, stoi
#include <vector> // vector
#include <algorithm> // min, max, swap, sort, reverse, lower_bound, upper_bound
#include <utility> // pair, make_pair
#include <tuple> // tuple, make_tuple
#include <map> // map
#include <queue> // queue, priority_queue
#include <set> // set
#include <stack> // stack
#include <deque> // deque
#define ll long long
#define ALL(x) (x).begin(),(x).end()
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define vi vector<int>
#define vvi vector<vector<int>>
using namespace std;

class UnionFind {
private:
    vector<vector<int>> Graph; // {ID,Number,ParentID,Height}

    void flatten(int x, int rootid) {
        int parentid = Graph[x][0];
        while (Graph[parentid][0] != Graph[parentid][2]) {
            Graph[parentid][2] = rootid;
            int next = Graph[parentid][2];
            parentid = next;
        }
    }
public:
    void push(int x) {
        int i = Graph.size();
        Graph.push_back({ i,x,i,1 });
    }
    void unite(int x, int y) {
        int hx = Graph[root(x)][3];
        int hy = Graph[root(y)][3];
        if (hx < hy) {
            Graph[root(x)][2] = root(y);
            flatten(x, root(y));
        }
        else if(hx == hy){
            Graph[root(y)][2] = root(x);
            Graph[root(x)][3]++;
            flatten(y, root(x));
        }
        else {
            Graph[root(y)][2] = root(x);
            flatten(y, root(x));
        }
    }
    bool same(int x, int y) {
        return root(x) == root(y);
    }

   
    int root(int x) {
        int parentid = Graph[x][0];
        while (Graph[parentid][0] != Graph[parentid][2]) {
            int next = Graph[parentid][2];
            parentid = next;
        }
       
        return parentid;
    }
};

int main() {
    int n; cin >> n;
    vector<vector<int>> edges; // {weight, from_id, to_id} both 0-indexed
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            int w; cin >> w;
            if (w == -1) {
                continue;
            }
            else {
                vector<int> temp_edge = { w,i,j };
                edges.push_back(temp_edge);
            }
        }
    }

    sort(ALL(edges));
    int n_edges = 0;
    int TotalCost = 0;

    UnionFind uf;
    for (int i = 0; i < n; i++) uf.push(i);

    for (int i = 0; n_edges < n - 1; i++) {
        vector<int> edge = edges[i];
       
        if (uf.root(edge[1]) != uf.root(edge[2])) {
            uf.unite(edge[1], edge[2]);
            n_edges++;
            TotalCost += edge[0];
        }
    }


    cout << TotalCost << endl;
}

C++: #include<iostream>
using namespace std;
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, N[MAX][MAX];

int prim()
{
  int u,minv;
  int d[MAX],p[MAX],color[MAX];
  int i=0;
  int v=0;
  int j=0;

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

  d[0] = 0;

  while(1)
    {
      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 && N[u][v]!=INFTY)
	    {
	      if(d[v]>N[u][v])
		{
		  d[v] = N[u][v];
		  p[v] = u;
		  color[v] = GRAY;
		}
	    }
	}
    }
  int sum=0;
  for(i=0;i<n;i++)
    {
      if(p[i]!=-1)
	{
	  sum += N[i][p[i]];
	}
    }

      return sum;
    }

  int main()
  {
    int i=0;
    int j=0;
    cin>>n;

    for(i=0;i<n;i++)
      {
	for( j=0 ; j<n ; j++ )
	  {
	    int e;
	    cin>>e;
	    N[i][j] = (e == -1)? INFTY : e;
	  }
      }

    cout<<prim()<<endl;

    return 0;
  }