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>
#define MAX 1000000000;
using namespace std;

int main(void)
{
	int n, now = 0, flag[101] = {};
	long ans = 0, an[100][100], min[100];

	cin >> n;

	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++)
			cin >> an[i][j];
		min[i] = MAX;
	}

	do {
		long key = MAX, key_i;
		flag[now] = 1;
		flag[100] = 0;

		for (int i = 0; i < n; i++) {
			if (an[now][i] != -1 && an[now][i] < min[i] && flag[i] == 0)
				min[i] = an[now][i];
			if (flag[i] == 0 && min[i] < key) {
				key = min[i];
				key_i = i;
			}
			if (flag[i] == 1)
				flag[100]++;
		}

		now = key_i;
		ans += key;

	} while (flag[100] != n - 1);

	cout << ans << endl;

	return 0;
}

C++: #include <iostream>
#include <vector>
#include <stack>
#include <queue>
#include <bitset>
#include <utility>
#include <functional>
#include <numeric>

template <class T>
using vec = std::vector<T>;

constexpr int nil = -1;
constexpr int inf = 1 << 30;

struct Edge
{
    int to;
    int cost;
};

int prim(const vec<vec<Edge>>& edges)
{
    const int n = edges.size();
    vec<bool> reached(n);
    vec<int> parents(n, nil);
    vec<int> distances(n, inf);

    distances[0] = 0;

    while (true)
    {
        int minCost {inf};
        int u {};
        
        for (int i = 0; i < n; ++i)
        {
            if (!reached[i] && distances[i] < minCost)
            {
                u = i;
                minCost = distances[i];
            }
        }
        
        if (minCost == inf)
            break;

        reached[u] = true;

        for (auto e : edges[u])
        {
            if (!reached[e.to])
            {
                if (e.cost < distances[e.to])
                {
                    distances[e.to] = e.cost;
                    parents[e.to] = u;
                }
            }
        }
    }

    int sum = std::accumulate(distances.begin(), distances.end(), 0);

    return sum;
}

void solve()
{
    using namespace std;
    int n;
    cin >> n;

    //隣接リスト形式
    vec<vec<Edge>> edges(n);

    for (int i = 0; i < n; ++i)
    {
        for (int j = 0, cost; j < n; ++j)
        {
            cin >> cost;
            if (cost != -1)
                edges[i].push_back({j, cost});
        }
    }

    cout << prim(edges) << endl;
}

int main()
{
    solve();
    return 0;
}

C++: #include<stdio.h>

#define MAX 1000
#define INFINITY 10000

int n;
int M[MAX][MAX], color[MAX], d[MAX], p[MAX];

enum {
    WHITE,
    GRAY,
    BLACK
};

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

    while (true) {
        int min = INFINITY;
        int target = -1;
        for (int i = 0; i < n; ++i) {
            if (d[i] < min && color[i] != BLACK) {
                min = d[i];
                target = i;
            }
        }
        if (target == -1) break;

        color[target] = BLACK;

        for (int v = 0; v < n; ++v) {
            if (color[v] != BLACK && M[target][v] != -1) {
                if(M[target][v] < d[v]) {
                    d[v] = M[target][v];
                    p[v] = target;
                    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) {
            scanf("%d", &M[i][j]);
        }
    }
    printf("%d\n", prim());
    return 0;
}

C++: #include <iostream>
#include <stack>
#include <string>
#include <vector>
using namespace std;
static const int Max = 100;
static const int Infinit = 1000000; // ??????
static const int Nil = -1; // 

enum Status {
	Not_Searched = 0,
	Searched = 1,
	Mts = 2 
};

int n;
int M[Max][Max] = {};
Status st[Max] = {}; 
int min_cost[Max] = {}; // i ??????????????????????????§???????°??????????
int parent[Max] = {}; // ??????index

int prim_search() {
	for (int i = 0; i < n; i++) {
		min_cost[i] = Infinit;
		parent[i] = -1;
		st[i] = Not_Searched;
	}
	min_cost[0] = 0;

	while (1) {
		int mincost = Infinit;
		int mincost_idx = -1;

		for (int i = 0; i < n; i++) {
			if (st[i] != Mts && min_cost[i] < mincost) {
				mincost = min_cost[i];
				mincost_idx = i;
			}
		}
		if (mincost_idx == -1) {
			break;
		}
		st[mincost_idx] = Mts;
		for (int i = 0; i < n; i++) {
			if (st[i] != Mts && M[mincost_idx][i] != Infinit) {
				if (min_cost[i] > M[mincost_idx][i]) {
					min_cost[i] = M[mincost_idx][i];
					parent[i] = mincost_idx;
					st[i] = Searched;
				}
			}
		}
	}
	int sum = 0;
	for (int i = 0; i < n; i++) {
		if (parent[i] != Nil) sum += M[i][parent[i]];
	}
	return sum;
}

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

	cout << prim_search() << endl;
	return 0;
}