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<bits/stdc++.h>

std::vector<int> color;
std::vector<std::vector<int>> M;
std::vector<int> d;
std::vector<int> p;
int INF = 1001001001;
int sum = 0;

int prim(int n){
    for(int i = 0; i < n; i++){
        color[i] = 0;
        d[i] = INF;
    }

    d[0] = 0;
    p[0] = -1;

    while(true){
        int u;
        int mincost = INF;
        for(int i = 0; i < n; i++){
            if(color[i] != 1 && d[i] < mincost){
                mincost = d[i];
                u = i;
            }
        }

        if(mincost == INF){
            break;
        }

        color[u] = 1;

        for(int i = 0; i < n; i++){
            if(color[i] != 1 && M[u][i] != -1){
                if(M[u][i] < d[i]){
                    d[i] = M[u][i];
                }
            }
        }

    }
    
    for(int i = 0; i < d.size(); i++){
        sum += d[i];
    }

    return sum;
}

int main(void){
    int n;
    std::cin >> n;

    color.resize(n);
    M.resize(n, std::vector<int>(n));
    d.resize(n);
    p.resize(n);

    for(int i = 0; i < n; i++){
        for(int j = 0; j < n; j++){
            std:: cin >> M[i][j];
        }
    }
    
    
    

    std::cout << prim(n) << std::endl;

    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

int n;
int a[101][101];

set<int>s,t;

int main(){
	s = set<int>();
	t = set<int>();
	scanf("%d",&n);
	for(int i = 0; i < n; i++){
		for(int j = 0; j < n; j++){
			scanf("%d",&a[i][j]);
		}
		if(i == 0){
			t.insert(i);
		}
		else{
			s.insert(i);
		}
	}
	int ans = 0;
	while(!s.empty()){
		int min_s=-1,min_t=-1,min_w=10000000;
		for(set<int>::iterator sit = s.begin(); sit != s.end(); sit++){
			for(set<int>::iterator tit = t.begin(); tit != t.end(); tit++){
				if(a[*sit][*tit] != -1 && a[*sit][*tit] < min_w){
					min_s = *sit;
					min_t = *tit;
					min_w = a[*sit][*tit];
				}
			}
		}
		s.erase(min_s);
		t.insert(min_s);
		ans += min_w;
	}
	printf("%d\n",ans);
	
	return 0;
}

C++: #include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (n); ++i)
using namespace std;
typedef float fl;
typedef long long ll;

static const int MAX = 100;
static const int INF = (1<<21);
static const int WHITE = 0;
static const int GRAY = 1;
static const int BLACK = 2;

int n, M[MAX][MAX];

int prim() {
    int u, minv;
    int d[MAX], p[MAX], color[MAX];

    rep(i,n) {
        d[i] = INF;
        p[i] = -1;
        color[i] = WHITE;
    }
    d[0] = 0;

    while(1){
        minv = INF;
        u = -1;
        rep(i,n) {
            if( minv > d[i] && color[i] != BLACK) {
                u = i;
                minv = d[i];
            }
        }
        if (u == -1) break;
        color[u] = BLACK;
        rep(v,n) {
            if (color[v] != BLACK && M[u][v] != INF) {
                if ( d[v] > M[u][v]){
                    d[v] = M[u][v];
                    p[v] = u;
                    color[v] = GRAY;
                }
            }
        }
    }
    int sum = 0;
    rep(i,n) {
        if (p[i] != -1) sum += M[i][p[i]];
    }
    return sum;
}

int main(){
    cin >> n;
    rep(i,n){
        rep(j,n){
            int e; cin >> e;
            M[i][j] = (e == -1)? INF : e;
        }
    } 
    cout << prim() << endl;
    return 0;
}

C++: #include <stdio.h>
#define MAX 1 << 22
int map[1000][1000];
int n, minval = 999999;

int prim()
{
    int sum = 0;
    int d[1000], fathernode[1000], color[1000];;
    for(int i = 0; i < n; i ++)
        d[i] = fathernode[i] = MAX;
    d[0] = 0;
    while(true)
    {
        int ch = -1, min = MAX;
        for(int i = 0; i < n; i ++)
        {
            if(min > d[i] && color[i] == 0) //选取未被染色的点
            {
                ch = i;
                min = d[i];
            }
        }
        if(ch == -1) //选完了，跳出
            break;
        color[ch] = 1;//选取这个点后染色
        for(int j = 0; j < n; j ++)//选取V-T中与T距离最近的节点
        {
            if(d[j] > map[ch][j] && color[j] == 0)
            {
                d[j] = map[ch][j];
                fathernode[j] = ch;//j节点的父节点是ch
            }
        }
    }
    //选取完毕，开始加起来
    for(int k = 0; k < n; k ++)//只加被染色（即被选中的）节点
    {
        if(fathernode[k] != MAX)//只加上有父节点的点
            sum += map[k][fathernode[k]];
    }
    return sum;
}

int main(void)
{
    scanf("%d", &n);
    for(int i = 0; i < n; i ++)
        for(int j = 0; j < n; j ++)
        {
            int t;
            scanf("%d", &t);
            map[i][j] = (t == -1 ? MAX : t);
        }
    printf("%d\n", prim() );
    return 0;
}