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>
#include<cstring>
#include<algorithm>
#include<cstdio>
using namespace std;
const int MAXN = 100 + 4;
const int INF = 0x3f3f3f3f;
const int WHITE = 0;
const int BLACK = 1;
const int GRAY = 2;
int G[MAXN][MAXN];
int d[MAXN], p[MAXN], status[MAXN];
int N;

int prim()
{
    int u, min_d;
    memset ( p, -1, sizeof(p) );
    memset ( status, WHITE , sizeof(status) );
    d[0] = 0;
    for ( int i = 1; i <= N; i++ ) d[i] = INF;

    while ( true ) {
        min_d = INF;
        u = -1;
        for ( int i = 0; i < N; i++ ) {
            if ( min_d > d[i] && status[i] != BLACK ) {
                u = i;
                min_d = d[i];
            }
        }

        if ( u == -1 ) break;
        status[u] = BLACK;
        for ( int v = 0; v < N; v++ ) {
            if ( G[u][v] != INF && status[v] != BLACK ) {
                if ( d[v] > G[u][v] ) {
                    d[v] = G[u][v];
                    p[v] = u;
                    status[v] = GRAY;
                }
            }
        }
    }
    int sum = 0;
    for ( int i = 0; i < N; i++ )
        if ( p[i] != -1 ) sum += G[i][p[i]];

    return sum;
}

int main()
{
    int e;
    cin>>N;
    for ( int i = 0; i < N; i++ ) {
        for ( int j = 0; j < N; j++ ) {
            cin>>e;
            G[i][j] = e == -1 ? INF : e;
        }
    }
    int ans = prim();
    cout<<ans<<endl;
}

C++: #define _CRT_SECURE_NO_WARNINGS
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<string>
#include<algorithm>
#include<cmath>
#include<queue>
#include<vector>
#include<stack>
using namespace std;

#define REP(i,n) for(int i=0;(i)<(n);(i)++)
#define ll long long

static const int INFTY = (1 << 25);

int G[100][100], n;

int makeMST() {
	bool mask[100][100];//MAST????????¨???????????¨???????????¨?????????1
	REP(i, n) {
		REP(j, n) mask[i][j] = 0;
	}

	bool color[100];//MST?????\??£??????1
	REP(i, n) color[i] = 0;

	color[0] = 1;//?????????????????¨??????0?????\??????

	REP(k, n - 1) {
		int minv = INFTY;
		int mNodej = -1;
		int mNodei = -1;
		REP(i, n) {
			if (color[i] == 1) {
				REP(j, n) {
					if (G[i][j] != -1) {
						if (color[j] == 0) {
							if (G[i][j] < minv) {
								minv = G[i][j];
								mNodei = i;
								mNodej = j;
							}
						}
					}
				}
			}
		}
		mask[mNodei][mNodej] = 1;
		mask[mNodej][mNodei] = 1;
		color[mNodej] = 1;
	}

	REP(i, n) {
		REP(j, n) {
			if (mask[i][j] == 0) G[i][j] = -1;
		}
	}

	int ans = 0;
	REP(i, n) {
		REP(j, n) {
			if (G[i][j] != -1) ans += G[i][j];
		}
	}

	return ans / 2;
}

int main() {
	cin >> n;

	REP(i, n) {
		REP(j, n) G[i][j] = INFTY;
	}
	REP(i, n) {
		REP(j, n) cin >> G[i][j];
	}

	cout << makeMST() << endl;


	return 0;
}

C++: #include <iostream>
#include <algorithm>
#define N 100
#define INF 2000000000
#define WHITE 0
#define GRAY 1
#define BLACK 2
using namespace std;

int n;
int M[N][N],color[N],d[N],p[N];
   //M??????????????????color????¨??????¶???(0,1,2),d????????????????°???????????¨????
   //p???MST???????°???¨?????¨??????????????????????????????

int Prim() {
	for (int i = 0;i < n;i++) d[i] = INF;
	d[0] = 0;  //?????????????????????????????§???????????§????´???????????????????????????????
	/*p[0] = -1;  //?????????????????????????????????????????? (???????????£????????????)*/

	int u;
	while (true) {
		int min = INF;
		for (int i = 0;i < n;i++) {  //MST????§???????????????????????????????????±????
			if(color[i]!=BLACK&&min>d[i]){
				min = d[i];  //??´??°?????????????????°min?????°??????d[i]???????´??????????????????????...
				u = i;
			}
		}

	if (min == INF) break; //????????\?????????d[i]?????´??°??§???????????¨???????????¨

	color[u] = BLACK;

	for (int v = 0;v < n;v++) 
		if(color[v]!=BLACK&&M[u][v]!=-1)
			if (M[u][v] < d[v]) {
				d[v] = M[u][v];  //?????????????????????????±????
				/*p[v] = u;*/
				/*color[v] = GRAY;(????????????bcSSSP??§?????¢?´¢?????????????????????????????§?????????S)*/
			}

	}

	int sum = 0;
	for (int i = 0;i < n;i++) sum += d[i];  //d[i]???????¨????MST???????????????
	return sum;
		
}

int main() {
	cin >> n;
	for (int i = 0;i < n;i++)
		for (int j = 0;j < n;j++) {
			cin >> M[i][j];
			if (M[i][j] == -1) M[i][j] = INF;
		}

	cout<<Prim()<<endl;

}