Go: package main

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

var (
	n int
	M [MAX][MAX]int
)

const MAX = 100
const WHITE = 0
const GRAY = 1
const BLACK = 2

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	sc.Scan()
	n, _ = strconv.Atoi(sc.Text())

	for i := 0; i < n; i++ {
		for j := 0; j < n; j++ {
			sc.Scan()
			c, _ := strconv.Atoi(sc.Text())
			if c != -1 {
				M[i][j] = c
			} else {
				M[i][j] = math.MaxInt64
			}

		}
	}
	prim()
}

func prim() {
	var u int
	var minv int
	var d [MAX]int
	var p [MAX]int
	var color [MAX]int

	for i := 0; i < n; i++ {
		d[i] = math.MaxInt64
		p[i] = -1
		color[i] = WHITE
	}

	d[0] = 0

	for {
		minv = math.MaxInt64
		u = -1
		for 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 v := 0; v < n; v++ {
			if color[v] != BLACK && M[u][v] != math.MaxInt64 {
				if d[v] > M[u][v] {
					d[v] = M[u][v]
					p[v] = u
					color[v] = GRAY
				}
			}
		}
	}
	sum := 0
	for i := 0; i < n; i++ {
		if p[i] != -1 {
			sum += M[i][p[i]]
		}
	}
	fmt.Println(sum)

}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <functional>

using namespace std;
typedef pair<int, int> P;   //cost, to
static const int INF = 1e9;
static const int SIZE = 101;
int par[SIZE];
int rk[SIZE];
int d[SIZE];
int n;

struct edge
{
	int u, v, cost;
};

struct edge2
{
	int to, cost;
};

bool comp(const edge& e1, const edge& e2)
{
	return(e1.cost < e2.cost);
}

void init()
{
	for (int i = 0; i < SIZE; ++i)
	{
		par[i] = i;
		rk[i] = 0;
	}
}

int find(int x)
{
	if (x == par[x])
	{
		return(x);
	}
	else
	{
		return(par[x] = find(par[x]));
	}
}

void unite(int a, int b)
{
	int x = find(a);
	int y = find(b);
	if (x == y)
	{
		return;
	}
	if (rk[x] < rk[y])
	{
		par[x] = y;
	}
	else
	{
		par[y] = x;
		if (rk[x] == rk[y])
		{
			++rk[x];
		}
	}
}

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

int kruskal(vector<edge>& es)
{
	init();
	int res = 0;
	sort(es.begin(), es.end(), comp);
	for (int i = 0; i < es.size(); ++i)
	{
		edge e = es[i];
		if (!same(e.u, e.v))
		{
			unite(e.u, e.v);
			res += e.cost;
		}
	}
	return(res);
}

int prim(vector<edge2>* G)
{
	vector<bool> isUsed(n);
	for (int i = 0; i < n; ++i)
	{
		isUsed[i] = false;
	}
	priority_queue<P, vector<P>, greater<P> > pque;
	P p = make_pair(0, 0);
	pque.push(p);
	int res = 0;
	while (!pque.empty())
	{
		P p = pque.top();
		pque.pop();
		int v = p.second;
		if (isUsed[v])
		{
			continue;
		}
		isUsed[v] = true;
		res += p.first;
		for (int i = 0; i < G[v].size(); ++i)
		{
			edge2 e = G[v][i];
			pque.push(P(e.cost, e.to));
		}
	}
	return(res);
}


void solve()
{
	cin >> n;

// For Kruskal
	vector<edge> es;
	for (int i = 0; i < n; ++i)
	{
		for (int j = 0; j < n; ++j)
		{
			int cost;
			cin >> cost;
			if (cost != -1)
			{
				edge e;
				e.u = i;
				e.v = j;
				e.cost = cost;
				es.push_back(e);
			}
		}
	}
	cout << kruskal(es) << endl;

// For Prim
	//vector<edge2> G[SIZE];
	//for (int i = 0; i < n; ++i)
	//{
	//	for (int j = 0; j < n; ++j)
	//	{
	//		int cost;
	//		cin >> cost;
	//		if (j >= i && cost != -1)
	//		{
	//			edge2 e1, e2;
	//			e1.to = j;
	//			e1.cost = cost;
	//			e2.to = i;
	//			e2.cost = cost;
	//			G[i].push_back(e1);
	//			G[j].push_back(e2);
	//		}
	//	}
	//}
	//cout << prim(G) << endl;
}

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

C++: #include <bits/stdc++.h>
#define For(i, a, b) for(int (i)=(a); (i)<(b); ++(i))
#define rFor(i, a, b) for(int (i)=(a)-1; (i)>=(b); --(i))
#define rep(i, n) For((i), 0, (n))
#define rrep(i, n) rFor((i), (n), 0)
#define fi first
#define se second
using namespace std;
typedef long long lint;
typedef pair<int, int> pii;

typedef struct UnionFindTree{
  vector<int> par;
  vector<int> treerank;
  
  UnionFindTree(int n){
    par.resize(n); treerank.resize(n);
    rep(i, n) par[i] = i, treerank[i] = 0;
  }
  
  int find(int x){
    if(par[x] == x) return x;
    return par[x] = find(par[x]);
  }
  
  void unite(int x, int y){
    x = find(x); y = find(y);
    if(x == y) return;
    if(treerank[x] < treerank[y]) swap(x, y);
    if(treerank[x] == treerank[y]) treerank[x]++;
    par[y] = x;
    return;
  }
  
  bool same(int x, int y){
    return find(x) == find(y);
  }
}UF;
 
template<class T> struct edge{
    int from, to; T cost;
    edge(int f, int t, T c){
        from = f; to = t; cost = c;
    }
};
 
template<class T> struct Graph{
    vector<vector<edge<T>>> G;
    int n;
    
    Graph(int n_){
        n = n_;
        G.resize(n);
    }
    
    void add_edge(int f, int t, T c){
        G[f].emplace_back(f, t, c);
    }
    
    vector<T> dijkstra(int s){
        using P = pair<T, int>;
        priority_queue<P, vector<P>, greater<P>> que;
        vector<T> d(n, numeric_limits<T>::max());
        d[s] = 0;
        que.push(P((T)0, s));
        while(!que.empty()){
            P p = que.top(); que.pop();
            int v = p.second;
            if(d[v] < p.first) continue;
            for(edge<T> &e : G[v]){
                if(d[e.to] > d[v] + e.cost){
                    d[e.to] = d[v] + e.cost;
                    que.push(P(d[e.to], e.to));
                }
            }
        }
        return d;
    }
    
    T kruskal(){
        vector<edge<T>> E;
        rep(i, n)for(edge<T> &e: G[i]) E.push_back(e);
        sort(E.begin(), E.end(), [](const edge<T> &e1, const edge<T> &e2){return e1.cost < e2.cost;});
        UF uf(n);
        T res = 0;
        for(edge<T> &e: E){
            if(!uf.same(e.from, e.to)){
                uf.unite(e.from, e.to);
                res += e.cost;
            }
        }
        return res;
    }
    
    vector<int> toposo(){
        vector<int> ret(n), in(n, 0);
        rep(i, n)for(edge<T> &e: G[i]) ++in[e.to];
        int cur = 0;
        stack<int> st;
        rep(i, n)if(!in[i]) st.push(i);
        while(!st.empty()){
            int v = st.top(); st.pop();
            ret[cur++] = v;
            for(edge<T> &e: G[v]){
                --in[e.to];
                if(!in[e.to]) st.push(e.to);
            }
        }
        return ret;
    }
};

int main(){
    int n;
    scanf("%d", &n);
    Graph<int> g(n);
    rep(i, n)rep(j, n){
        int a;
        scanf("%d", &a);
        if(a >= 0) g.add_edge(i, j, a);
    }
    printf("%d\n", g.kruskal());
}

C++: #include<iostream>
#include<algorithm>
#include<vector>
using namespace std;
struct Node{
	int f,e,w;
};
inline bool cmp(Node a,Node b){
	return a.w<b.w;
}
int bc[100+30];
int bcj(int x){
    if(bc[x]==x) return bc[x];
    return bc[x]=bcj(bc[x]);
}
int ans,n;
vector<Node> edl;
int main(){
	cin>>n;
	for(int i=0;i<n;++i) bc[i]=i;
	for(int i=0;i<n;++i){
		for(int j=0;j<n;++j){
			Node x;
			x.f=i;x.e=j;
			cin>>x.w;
			if(x.w!=-1) edl.push_back(x);
		}
	}
	sort(edl.begin(),edl.end(),cmp);
	int tot=0,xi=0;
	while(tot<n-1){
		int a=edl[xi].f,b=edl[xi].e;
		if(bc[bcj(a)]!=bcj(b)){
			bc[bcj(a)]=bcj(b);
			++tot;
			ans+=edl[xi].w;
		}
		++xi;
	}
	cout<<ans<<endl;
	return 0;
}