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 <bits/stdc++.h>

using namespace std;

typedef long long int ll;
typedef pair<ll, ll> pll;

#define FOR(i, n, m) for(ll (i)=(m);(i)<(n);++(i))
#define REP(i, n) FOR(i,n,0)
#define OF64 std::setprecision(10)

const ll MOD = 1000000007;
const ll INF = (ll) 1e15;

struct UnionFind {
    UnionFind(int n) {
        rank.assign(n, 0);
        parent.resize(n);
        for (int i = 0; i < n; ++i) {
            parent[i] = i;
        }
    }

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

    bool same(int x, int y) {
        return find(x) == find(y);
    }

    void unit(int x, int y) {
        x = find(x);
        y = find(y);
        if (x == y)
            return;

        if (rank[x] < rank[y]) {
            parent[x] = y;
        }
        else {
            parent[y] = x;
            if (rank[x] == rank[y])
                rank[x]++;
        }
    }

    vector<int> rank;
    vector<int> parent;
};

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    ll N;
    cin >> N;
    priority_queue<tuple<pll, ll>, vector<tuple<pll, ll>>, function<bool(tuple<pll, ll>, tuple<pll, ll>)>> q(
            [](tuple<pll, ll> a, tuple<pll, ll> b) {
                return get<1>(a) > get<1>(b);
            });
    REP(i, N) {
        REP(j, N) {
            ll a;
            cin >> a;
            if (a < 0)
                continue;
            q.push(make_tuple(pll(i, j), a));
        }
    }
    UnionFind uf(N);
    ll ans = 0;
    while (!q.empty()) {
        auto t = q.top();
        q.pop();
        pll p = get<0>(t);
        if (uf.same(p.first, p.second))
            continue;
        uf.unit(p.first, p.second);
        ans += get<1>(t);
    }
    cout << ans << endl;
    return 0;
}

C++: #include <stdio.h>
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 d[n], p[n];
    int color[n];
    
    for(int i=0; i<n; i++) {
        color[i] = WHITE;
        d[i] = INFTY;
        p[i] = -1;
    }
    
    d[0] = 0;
    
    while(1) {
        int u;
        int mincost = INFTY;
        for (int i=0; i<n; i++) {
            if (color[i] != BLACK && d[i] < mincost) {
                mincost = d[i];
                u = i;
            }
        }
        
        if (mincost == INFTY) break;
        
        color[u] = BLACK;
        
        for (int v=0; v<=n; v++) {
            if (color[v] != BLACK && M[u][v] != INFTY) {
                if(M[u][v] < d[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[p[i]][i];
    }
    return sum;
}

int main() {
    scanf("%d", &n);
    for (int i=0; i<n; i++) {
        for (int j=0; j<n; j++) {
            int tmp;
            scanf("%d", &tmp);
            M[i][j] = (tmp == -1) ? INFTY : tmp;
        }
    }
    
    int sum = prim();
    printf("%d\n", sum);
    
    return 0;
}

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]; 
    // Tに属する頂点とV-Tに属する頂点をつなぐ辺の中で、重みが最小の辺の重みを記録
    // MSTにおける頂点vの親を記録
    // 訪問状態を記録

    for(int i=0; i<n; i++){ // 全ての頂点に関して、colorをwhite、dをinftyに初期化
        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++){ // 最小の辺を探索
            if(minv > d[i] && color[i] != BLACK){
                u=i;
                minv = d[i];
            }
        }
        if(u==-1) break;
        for(int v=0; v<n; v++){
            if(color[v] != BLACK && M[u][v] != INFTY){ // 未訪問かつuからvに辺があるとき
                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(int argc, char const *argv[])
{
    cin >> n;
    for(int i=0; i<n;i++){
        for(int j=0;j<n;j++){
            int e;
            cin >> e;
            M[i][j] = (e==-1) ? INFTY : e;

        }
    }

    cout << prim() << endl;

    return 0;
}

C++: #include <stdio.h>
#include <iostream>
#include <string>
#include <sstream>
#include <stack>
#include <algorithm>
#include <math.h>
#include <queue>
#include <string.h>
#include <map>
#include <set>
#include <cstdlib>

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

char ToUpper(char cX) { return toupper(cX); }
char Tolower(char cX) { return tolower(cX); }

const long long INF = 1LL << 60;
const long long MOD = 1000000007;

using namespace std;
typedef unsigned long long ull;
typedef  long long ll;



vector<vector<ll>> dp;

class UnionFind {
public:
	vector<ll> Parent;
	UnionFind(ll N) {
		Parent = vector<ll>(N, -1);
	}
	ll root(ll A) {
		if (Parent[A] < 0) {
			return A;
		}
		return Parent[A] = root(Parent[A]);
	}

	ll size(ll A) {
		return -Parent[root(A)];
	}

	bool connect(ll A, ll B) {
		A = root(A);
		B = root(B);
		if (A == B) {
			return false;
		}
		if (size(A) < size(B)) {
			swap(A, B);
		}

		Parent[A] += Parent[B];
		Parent[B] = A;

		return true;
	}

};

class Edge {
public:
	ll source, target, cost;
	Edge(ll source = 0, ll target = 0, ll cost = 0) :
		source(source), target(target), cost(cost) {}
	bool operator<(const Edge &e)const {
		return cost < e.cost;
	}
};

ll kuraskal(ll N, vector<Edge> &edges) {
	ll totalCost = 0;
	sort(edges.begin(), edges.end());
	UnionFind uni(N + 1);
	ll source, target;
	for (ll i = 0; i < edges.size(); i++) {
		Edge e = edges[i];
		if (uni.root(e.source) != uni.root(e.target)) {
			totalCost += e.cost;
			uni.connect(e.source, e.target);
		}
	}
	return totalCost;
}




int main()
{
	ll n;
	cin >> n;
	vector<vector<ll>> a;
	a.resize(n);
	for (int i = 0; i < n; i++) {
		a[i].resize(n);
	}
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			cin >> a[i][j];
		}
	}
	vector<Edge> edges;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < n; j++) {
			if (a[i][j] != -1) {
				edges.push_back(Edge(i, j, a[i][j]));
			}
		}
	}

	cout << kuraskal(n, edges) << endl;
	return 0;
}