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 <cstdio>
#include <iostream>
#include <queue>
#include <utility>

using namespace std;

const int NIL = 2001;
const int MAX = 100;
int G[MAX][MAX];
int color[MAX];
int W[MAX];
int n;

const int WHITE = 0;
const int GRAY = 1;
const int BLACK = 2;

void MST() {
	priority_queue<pair<int, int>> PQ;
	PQ.push(make_pair(0, 0));
	W[0] = 0;
	color[0] = BLACK;
	while (!PQ.empty()) {
		int u = PQ.top().second;
		PQ.pop();
		color[u] = BLACK;
		for (int i = 0; i < n; ++i) {
			if (G[u][i] != -1 && G[u][i] < W[i] && color[i] != BLACK) {
				PQ.push(make_pair(G[u][i] * -1, i));
				W[i] = G[u][i];
//				cout << u << ":" << i << ":" << G[u][i] << endl;
			}
		}
	}
}

int main() {

	scanf("%d", &n);
	for (int i = 0; i < n; ++i) {
		W[i] = NIL;
		color[i] = WHITE;
	}

	for (int i = 0; i < n; ++i) {
		for (int j = 0; j < n; ++j) {
			int a;
			scanf("%d", &a);
			G[i][j] = a;
		}
	}

	MST();

	cout << endl;

	int sum = 0;
	for (int i = 0; i < n; ++i) {
//		cout << i << ":" << W[i] << endl;
		sum += W[i];
	}
	cout << sum << endl;

	return 0;
}

C++: #include <iostream>
#include <vector>
#include <numeric>

using namespace std;

const int N = 100;

/*
// ????????????
Vnew = {x}???????????§ x ??? V ????????§??????????????????????????????????????????
Enew = {}
while (Vnew ??? V)
    Vnew ????????????????????? u ??¨ ????????????????????? v ????????¶?????????????°??????? (u, v) ??? E ????????????
    v ??? Vnew ????????????
    (u, v) ??? Enew ????????????
*/

enum Color : int {
    WHITE,
    GRAY,
    BLACK,
};

int main() {
    int A[N][N];

    int n;
    cin >> n;
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            int a;
            cin >> a;
            A[i][j] = a == -1 ? a : numeric_limits<int>::max();
        }
    }

    vector<int> color(n, WHITE);
    vector<int> d(n, numeric_limits<int>::max());
    vector<int> p(n, -1);

    d[0] = 0;
    while (1) {
        int minv = numeric_limits<int>::min();
        int 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 && A[u][v] != numeric_limits<int>::max()) {
                if (d[v] > A[u][v]) {
                    d[v] = A[u][v];
                    p[v] = u;
                    color[v] = GRAY;
                }
            }
        }
    }
    int sum;
    for (int i =0; i < n; ++i) {
        if (p[i] != -1)
            sum += A[i][p[i]];
    }
    cout << sum << endl;

    return 0;
}

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

static const int MAX_NUM  = 101;
static const int NO_VISIT  = 0;
static const int NOW_VISIT = 1;
static const int END_VISIT = 2;

void MPT(int *pElem, int num)
{
    int i;
    int mincost;
    int target;
    int parent[MAX_NUM];
    int cost[MAX_NUM];
    int state[MAX_NUM] = {0};
    int *tmp;
    int total = 0;

    for (i = 0; i < num; i++) {
        parent[i] = -1;
        cost[i]   = 0x7FFFFFFF;
    }

    cost[0] = 0;

    for (;;) {
        mincost = 0x7FFFFFFF;
        for (i = 0; i < num; i++) {
            if ((state[i] != END_VISIT) && (mincost > cost[i])) {
                target  = i;
                mincost = cost[i];
            }
        }
        if (mincost == 0x7FFFFFFF) {
            break;
        }

        state[target] = END_VISIT;
        total += mincost;

        tmp = pElem + target * MAX_NUM;
        for (i = 0; i < num; i++) {
            if ((state[i] != END_VISIT) && (*(tmp + i) < cost[i])) {
                cost[i]   = *(tmp + i);
                parent[i] = target;
                state[i]  = NOW_VISIT;
            }
        }
    }

    printf("%d\n", total);

    return;
}

int main(void)
{
    int i, j;
    int num;
    int tmp;
    int elem[MAX_NUM][MAX_NUM];

    scanf("%d", &num);
    for (i = 0; i < num; i++) {
        for (j = 0; j < num; j++) {
            scanf("%d", &tmp);
            elem[i][j] = (tmp != -1) ? tmp : 0x7FFFFFFF;
        }
    }

    MPT(&elem[0][0], num);

    return 0;
}

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

static const int N=100;
static const int INF=100000;

int n;
int M[N][N];

//初期化
int prim(){
  int u,minv;
  int d[N],p[N],color[N];
  for(int i=0;i<n;i++){
    d[i]=INF;
    p[i]=-1;
    color[i]=0;
  }
  
  d[0]=0;
  
  while(1){
    minv=INF;
    u=-1;
    for(int i=0;i<n;i++){
      if(minv>d[i]&&color[i] != 2){
        u = i;
        minv = d[i];
      }
    }
    if(u == -1) break;
    color[u] = 2;
    
    for(int v=0;v<n;v++){
      if(color[v] !=2 && M[u][v] != INF){
        if(d[v]>M[u][v]){
          d[v]=M[u][v];
          p[v]=u;
          color[v]=1;
        }
      }
    }
  }
  int sum=0;
  for(int i=0;i<n;i++){
    if(p[i] !=-1 ) sum += M[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;
      M[i][j]=(e== -1)? INF : e;
    }
  }
  
  cout<<prim()<<endl;
  return 0;
}