Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

type ElementKey int
type Node struct {
	key    ElementKey
	parent *Node
	rank   int
}

func newNode(key ElementKey) *Node {
	node := Node{}
	node.key = key
	node.parent = &node
	node.rank = 0

	return &node
}

func union(x, y *Node) {
	xSet := findSet(x)
	ySet := findSet(y)
	link(xSet, ySet)
}

func link(x, y *Node) {
	if x.rank > y.rank {
		y.parent = x
	} else {
		x.parent = y

		if x.rank == y.rank {
			y.rank++
		}
	}
}

func findSet(x *Node) *Node {
	if x != x.parent {
		x.parent = findSet(x.parent)
	}
	return x.parent
}

func newDisjointSets(nSets int) *DisjointSets {
	sets := DisjointSets{}
	sets.keyToNode = map[ElementKey]*Node{}
	for i := 0; i < nSets; i++ {
		key := ElementKey(i)
		sets.keyToNode[key] = newNode(key)
	}
	return &sets
}

type DisjointSets struct {
	keyToNode map[ElementKey]*Node
}

func (sets *DisjointSets) union(x, y ElementKey) {
	xNode := sets.keyToNode[x]
	yNode := sets.keyToNode[y]
	union(xNode, yNode)
}

func (sets DisjointSets) findSet(x ElementKey) *Node {
	xNode := sets.keyToNode[x]
	return findSet(xNode)
}

type Edge struct {
	i, j   int
	weight int
}

func main() {
	scanner.Split(bufio.ScanWords)

	n := nextInt()
	sets := newDisjointSets(n)
	edges := []Edge{}
	a := make([][]int, n)
	for i := 0; i < n; i++ {
		a[i] = make([]int, n)
		for j := 0; j < n; j++ {
			a[i][j] = nextInt()
			if a[i][j] >= 0 {
				e := Edge{
					i:      i,
					j:      j,
					weight: a[i][j],
				}
				edges = append(edges, e)
			}
		}
	}
	sort.Slice(edges, func(i, j int) bool { return edges[i].weight < edges[j].weight })
	totalWeight := 0
	for _, edge := range edges {
		i := ElementKey(edge.i)
		j := ElementKey(edge.j)
		if sets.findSet(i) != sets.findSet(j) {
			totalWeight += edge.weight
			sets.union(i, j)
		}
	}
	fmt.Println(totalWeight)
}

C++: #include <bits/stdc++.h>
using namespace std;
struct UnionFind{
    vector<int> parent;

    UnionFind(int n) : parent(n){
        for (int i = 0; i < n; i++)parent[i] = i;
    }

    int find(int e){
        if (parent[e] == e) return e;
        else return parent[e] = find(parent[e]);
    }

    void unite (int a, int b) {
        a = find(a);
        b = find(b);
        if (a == b) return;
        parent[a] = b;
    }

    bool same(int a, int b){
        return find(a) == find(b);
    }
};

int main() {
    vector<tuple<int, int, int>> a;
    int n, t, ans = 0;
    cin >> n;
    UnionFind uf(n);
    for(int i = 0; i < n; i++){
        for(int j = 0; j < n; j++){
            cin >> t;
            if (i <= j && t != -1) a.push_back(make_tuple(t,i,j));
        }
    }
    sort(a.begin(), a.end(), [](const tuple<int, int, int>& x, const tuple<int, int, int>& y) { return get<0>(x) < get<0>(y);} );
    for(auto e : a){
        if( !uf.same(get<1>(e), get<2>(e)) ){
            ans += get<0>(e);
            uf.unite(get<1>(e), get<2>(e));
        }
    }
    cout << ans << endl;
}

C++: #include <iostream>

const int MAX = 101;
const int NIL = -1; 
const int UPBOUND = (1 << 20);

using namespace std;

int prim(int adj_matrix[][MAX], int n, int u);


int main()
{
	int adj_matrix[MAX][MAX] = {0};		
	
	int n;
	cin >> n;

	for (int i = 1; i <= n; i++)
	{
		for (int j = 1; j <= n; j++)
		{
			int e; cin >> e;
			adj_matrix[i][j] = e == -1? UPBOUND: e;
		}
	}
	
//	for (int i = 0; i < n; i++)
//	{
//		for (int j = 0; j < n; j++)
//		{
//			cout << adj_matrix[i][j] << " ";
//		}
//		cout << endl;
//	}

	
	cout << prim(adj_matrix, n, 1) << endl;
} 


int prim(int adj_matrix[][MAX], int n, int u)
{
	int cost[MAX]; //记录最小权值
	int edge[MAX]; //记录选中的边
	
	
	for (int v = 1; v <= n; v++)
	{
		cost[v] = adj_matrix[u][v];
		edge[v] = u;
	}	 
	
	cost[u] = NIL; //排除u 
	
	while (true)
	{
		int w = NIL;
		int min = 3000; //权值上限 
		for (int v = 1; v <= n; v++)
		{
			if (cost[v] != NIL && cost[v] < min)
			{
				min = cost[v];
				w = v;
			} 
		}
		
		if (w == NIL) break;
		
		cost[w] = NIL;
		
		//更新最小权值数组 
		for (int v = 1; v <= n; v++)
		{
			if (cost[v] > adj_matrix[w][v] && adj_matrix[w][v] != NIL)
			{
				cost[v] = adj_matrix[w][v];
				edge[v] = w;			
			}
		}
				
	}
	
	
	//求生成树的权值
	int sum = 0;
	for (int v = 1; v <= n; v++)
	{
		if (v != u)
		sum += adj_matrix[v][edge[v]];
	} 
	
	return sum;	
	
}

C++: #include <iostream>
#include <limits>

constexpr int MAXN = 100;
int G[MAXN][MAXN];
int P[MAXN];
int D[MAXN];
int C[MAXN];

int main(){
    int n;
    std::cin >> n;
    for(int i = 0; i < n; i++) {
        for(int j = 0; j < n; j++) {
            std::cin >> G[i][j];
            if (G[i][j] == -1) {
                G[i][j] = std::numeric_limits<int>::max();
            }
        }
    }

    for (int i = 0; i < n; i++) {
        D[i] = std::numeric_limits<int>::max();
    }

    D[0] = 0;
    P[0] = -1;
    while (true) {
        int m;
        int mincost = std::numeric_limits<int>::max();
        for (int i = 0; i < n; i++) {
            if ((C[i] == 0) && (D[i] < mincost)) {
                mincost = D[i];
                m = i;
            }
        }

        if (mincost == std::numeric_limits<int>::max()) {
            break;
        }

        C[m] = 1;
        D[m] = mincost;

        for (int i = 0; i < n; i++) {
            if ((G[m][i] < std::numeric_limits<int>::max()) && (C[i] == 0) && (G[m][i] < D[i])) {
                D[i] = G[m][i];
            }
        }
    }


    int sum = 0;
    for (int i = 0; i < n; i++) {
        if (D[i] != std::numeric_limits<int>::max()) {
            sum += D[i];
        }
    }
//    for (int i = 0; i < n; i++) {
//        std::cout << i << ": " << D[i] << std::endl;
//    }
    std::cout << sum << std::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, M[MAX][MAX];

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

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

	d[0] = 0;
	while (true) {
		minv = INFTY;
		u = -1;
		for (int 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 (int v = 0; v < n; v++) {
			if (color[v] != BLACK && M[u][v] != INFTY) {
				if (d[v] > M[u][v]) {
					d[v] = M[u][v];
					p[v] = u;
					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() {
	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] = INFTY;
		}
	}
	cout << prim() << '\n';
	return 0;
}