Go: package main

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

const (
	MAX_N      = 200
	MAX_WEIGHT = 2000
)

type Edge struct {
	start  int
	end    int
	weight int
}

type ShortestInfo struct {
	edges    []Edge
	shortest int
}

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)

	n, err := nextInt(sc)
	if err != nil {
		panic(err)
	}

	shortestList := make([]ShortestInfo, n)
	for i := 1; i < n; i++ {
		shortestList[i] = ShortestInfo{edges: []Edge{}, shortest: MAX_N*MAX_WEIGHT + 1}
	}

	adjacentMatrix := make([][]int, n)
	for i := 0; i < n; i++ {
		adjacentMatrix[i] = make([]int, n)
		for j := 0; j < n; j++ {
			v, err := nextInt(sc)
			if err != nil {
				panic(err)
			}
			adjacentMatrix[i][j] = v
		}
	}

	mst([]int{1}, shortestList, adjacentMatrix)
	fmt.Println(calculateMinWeight(shortestList))
}

func calculateMinWeight(shortestList []ShortestInfo) int {
	visitedList := make([]bool, len(shortestList))
	visitedList[0] = true
	sumWeight := 0
	for !visitedAll(visitedList) {
		shortestInfo := shortestList[0]
		shortestList = shortestList[1:]
		edges := shortestInfo.edges
		for _, edge := range edges {
			if !visitedList[edge.end-1] {
				visitedList[edge.end-1] = true
				sumWeight += edge.weight
			}
		}
	}
	return sumWeight
}

func visitedAll(visitedList []bool) bool {
	for _, v := range visitedList {
		if !v {
			return false
		}
	}
	return true
}

func mst(nodeStack []int, shortestList []ShortestInfo, adjacentMatrix [][]int) {
	for len(nodeStack) > 0 {
		currentNode := nodeStack[0]
		nodeStack = nodeStack[1:]
		currentShortest := getShortest(currentNode, shortestList)
		adjacents := findAdjacents(currentNode, adjacentMatrix)
		for i := 0; i < len(adjacents); i++ {
			adj := adjacents[i]
			s := getShortest(adj.end, shortestList)
			if currentShortest.shortest+adj.weight < s.shortest || (currentShortest.shortest+adj.weight == s.shortest && len(currentShortest.edges)+1 > len(s.edges)) {
				setShortest(adj.end, adj, currentShortest, shortestList)
				nodeStack = append(nodeStack, adj.end)
			}
		}
	}
}

func getShortest(node int, shortestList []ShortestInfo) ShortestInfo {
	return shortestList[node-1]
}

func setShortest(node int, edge Edge, currentShortest ShortestInfo, shortestList []ShortestInfo) {
	newEdges := make([]Edge, len(currentShortest.edges)+1)
	for i := 0; i < len(newEdges)-1; i++ {
		newEdges[i] = currentShortest.edges[i]
	}
	newEdges[len(newEdges)-1] = edge
	shortestInfo := ShortestInfo{edges: newEdges, shortest: currentShortest.shortest + edge.weight}
	shortestList[node-1] = shortestInfo
}

func findAdjacents(currentNode int, adjacentMatrix [][]int) []Edge {
	adjacents := adjacentMatrix[currentNode-1]
	edges := []Edge{}
	for i, v := range adjacents {
		if v > 0 {
			edges = append(edges, Edge{start: currentNode, end: i + 1, weight: v})
		}
	}
	return edges
}

func nextInt(sc *bufio.Scanner) (int, error) {
	sc.Scan()
	return strconv.Atoi(sc.Text())
}

C++: #include <iostream>
#include <cstdio>
#include <algorithm>
#include <utility>
#include <vector>
using namespace std;
#define reps(i,s,n) for(int i = s; i < n; i++)
#define rep(i,n) reps(i,0,n)
#define fi first
#define se second
typedef long long ll;
typedef vector<ll> vec;
typedef vector<vec> mat;
typedef pair<ll,ll> pa; 
#define mp make_pair;


int N,M,H,W,K,A,B;
string S;
string alpha = "abcdefghijklmnopqrstuvwxyz";
string ALPHA = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

int main() {
  cin>>N;
  mat a(N,vec(N));
  rep(i,N) rep(j,N) cin>>a.at(i).at(j);
  class union_find{
        vector<long> parent, tree_size;
    public:
        union_find(unsigned long _n) : parent(_n), tree_size(_n, 1){
            for(unsigned long i = 0; i < _n; ++i)parent[i] = i;
        }
        ll find(ll x){
            while(parent[x] != x)x = parent[x] = parent[parent[x]];
            return x;
        }
        void merge(ll a, ll b){
            a = find(a);
            b = find(b);
            if(tree_size[a] < tree_size[b])swap(a, b);
            tree_size[a] += tree_size[b];
            parent[b] = a;
            return;
        }
	bool same(ll a, ll b){
	    a = find(a);
	    b = find(b);
	    return a == b;
	}
  };
  union_find uf(N);
  vector<pair<ll,ll> > edge(0);
  rep(i,N){
    reps(j,i+1,N){
      if(a[i][j] != -1){
	edge.push_back(pa(a[i][j],i*N+j));
      }
    }
  }
  sort(edge.begin(),edge.end());
  ll anse=0,ans=0;
  rep(i,edge.size()){
    pa e = edge[i];
    if(uf.same(e.se/N,e.se%N)) continue;
    ans += e.fi;
    uf.merge(e.se/N,e.se%N);
    ++anse;
    if(anse == N-1) break;
  }
  if(anse<N-1) cout<<"impossible"<<endl;
  cout<<ans<<endl;
  //uf.mergeのように使う
  //ufはMAX_N+1でとるか、代入時に0-indexedにする
}

C++: #include <iostream>
using namespace std;
#define MAX 100
#define INFTY (1<<21)
#define WHITE 0
#define GRAY 1
#define 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(1) {
        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++) {
            int e;
            cin >> e;
            if(e==-1) {
                M[i][j] = INFTY;
            }
            else {
                M[i][j] = e;
            }
        }
    }

    cout << prim() << endl;

    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
static const int INF   = (1 << 21);
static const int WHITE = 0;
static const int GRAY  = 1;
static const int BLACK = 2;
static const int NIL   = -1;

int n;

void prim(vector<vector<int>> &A) {
  vector<int> color(n, WHITE);
  vector<int> d(n, INF);
  vector<int> p(n, 0);
  d[0] = 0;
  p[0] = -1;

  while (true) {
    int mincost = INF;
    int u       = -1;

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

    for (int v = 0; v < n; v++) {
      if (color[v] != BLACK && A[u][v] != -1) {
        if (A[u][v] < d[v]) {
          d[v]     = A[u][v];
          p[v]     = u;
          color[v] = GRAY;
        }
      }
    }
  }
  int res = 0;
  for (int i = 0; i < n; i++) {
    res += d[i];
  }
  cout << res << endl;
}

int main() {
  cin >> n;
  vector<vector<int>> A(n, vector<int>(n));

  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      cin >> A[i][j];
    }
  }
  prim(A);
}