Go: package main

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

// Define disjoint-set forests

type vertexNo int

type vertex struct {
	number vertexNo
	rank int
	parent *vertex
}

func makeSet(number vertexNo) vertex {
	return vertex {
		number: number,
		rank: 0,
		parent: nil,
	}
}

func union(x, y *vertex) *vertex {
	return link(findSet(x), findSet(y))
}

func link(x, y *vertex) *vertex {
	if x.rank > y.rank {
		y.parent = x
		return x
	} else {
		x.parent = y
		if x.rank == y.rank {
			y.rank += 1
		}
		return y
	}
}

func findSet(v *vertex) *vertex {
	if v.parent == nil {
		return v
	}
	v.parent = findSet(v.parent)
	return v.parent
}

// Function to calculate minimum spanning tree

type matrix [][]int

type edge struct {
	one vertexNo
	two vertexNo
	weight int
}

const(
	MAX_VERTICES = 100
)

func newEdgesFromMatrix(matrix matrix) []edge {
	edges := make([]edge, 0, MAX_VERTICES * MAX_VERTICES)

	for i := 0; i < len(matrix); i++ {
		row := matrix[i]
		for j := i; j < len(row); j++ {
			if row[j] >= 0 {
				edge := edge{one: vertexNo(i+1), two: vertexNo(j+1), weight: row[j]}
				edges = append(edges, edge)
			}
		}
	}

	return edges
}

func solve(matrix matrix) int {
	nVertices := len(matrix)

	// initialize vertices
	vertices := make([]vertex, nVertices)
	for i := 0; i < nVertices; i++ {
		vertices[i] = makeSet(vertexNo(i+1))
	}

	// prepare edges sorted in the ascending order of weight
	edges := newEdgesFromMatrix(matrix)
	sort.Slice(edges, func(i, j int) bool {
		return edges[i].weight < edges[j].weight
	})

	unionCount := 0
	weightSum := 0
	for unionCount < nVertices-1 {
		for i := 0; i < len(edges); i++ {
			edge := edges[i]
			v1 := &vertices[edge.one-1]
			v2 := &vertices[edge.two-1]
			rep1 := findSet(v1)
			rep2 := findSet(v2)
			if rep1.number == rep2.number {
				continue
			}
			union(rep1, rep2)
			edges = append(edges[0:i], edges[i+1:]...)
			weightSum += edge.weight
			unionCount++
			break
		}
	}

	return weightSum
}

// main

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

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

	matrix := parseMatrix(n, sc)

	edges := newEdgesFromMatrix(matrix)
	sort.Slice(edges, func(i, j int) bool {
		return edges[i].weight < edges[j].weight
	})

	fmt.Println(solve(matrix))
}

func parseMatrix(n int, sc *bufio.Scanner) [][]int {
	matrix := make([][]int, n)
	for i := 0; i < n; i++ {
		matrix[i] = make([]int, n)
		for j := 0; j < n; j++ {
			x, err := nextInt(sc)
			if err != nil {
				panic(err)
			}
			matrix[i][j] = x
		}
	}
	return matrix
}

func nextText(sc *bufio.Scanner) string {
	sc.Scan()
	return sc.Text()
}

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

C++: #include<iostream>
using namespace std;
#define INFTY (1 << 30)

enum Color {
	WHITE,
	GRAY,
	BLACK,
};

void Prim(int verValWeight[][100], int verNum) {
	int distance[100];
	Color color[100];
	int parent[100];
	for (int i = 0; i < verNum; i++) {
		color[i] = WHITE;
		distance[i] = INFTY;
		parent[i] = -1;
	}
	distance[0] = 0;

	while (true) {
		int min_cost = INFTY;
		int subScript;
		for (int i = 0; i < verNum; i++) {
			if (color[i] != BLACK && distance[i] < min_cost) {
				min_cost = distance[i];
				subScript = i;
			}

			if (min_cost == INFTY)
				break; //end

			color[subScript] = BLACK;

			for (int i = 0; i < verNum; i++) {
				if (color[i] != BLACK && verValWeight[subScript][i] != INFTY) {
					if (verValWeight[subScript][i] < distance[i]) {
						distance[i] = verValWeight[subScript][i];
						parent[i] = subScript;
						color[i] = GRAY;
					}
				}
			}
		}
	}

	int weight_sum = 0;
	for (int i = 0; i < verNum; i++)
		if (parent[i] != -1)
			weight_sum += verValWeight[i][parent[i]];
	cout << weight_sum << endl;

}

int main() {
	int verNum;
	int verValWeight[100][100];
	int parent[100];

	cin >> verNum;
	for (int i = 0; i < verNum; i++)
		for (int j = 0; j < verNum; j++) {
			cin >> verValWeight[i][j];
			if (verValWeight[i][j] == -1) verValWeight[i][j] = INFTY;
		}

	Prim(verValWeight, verNum);

	return 0;
}

C++: #include <iostream>
//#include <string>
#include <algorithm>
using namespace std;
//#define int long long
//#define _DEBUG
class UnionFindTree{
    private:
    int P[10010];
    public:
    UnionFindTree(int N){
        for(int i=0; i<=N; i++) P[i] = i;
    }
    int root(int a){
        if (P[a] == a) return a;
        return (P[a] = root(P[a]));
    }
    bool is_same_set(int a, int b){
        return root(a) == root (b);
    }
    void unite(int a, int b){
        P[root(a)] = root(b);
    }
};
struct weight_edge{// (weight, (vert_a,vert_b))
    int w;
    int a;
    int b;
};
int n, k;
weight_edge we[10007];
bool com_we(weight_edge a, weight_edge b){
    return ((a.w == b.w)? a.a < b.a: a.w < b.w);
}
int solve(){
    if(n==0)return 0;
    sort(we, we+k, com_we);
    //for(int i=0; i<k; i++)printf("%d,%d,%d\n",we[i].w,we[i].a,we[i].b);
    auto uft = UnionFindTree(n);
    int i=0, r=0, l=0;
    while(l<n-1){
        if(!uft.is_same_set(we[i].a, we[i].b)){
            uft.unite(we[i].a, we[i].b);
            r += we[i].w;
            l++;
            //printf("%d;\tw:%d,\t(%d,%d)\n",l,we[i].w,we[i].a,we[i].b);
        }
        i++;
    }
    return r;
}
void get(){
    k = 0;
    for(int i=0; i<n; i++){
        for(int j=0; j<n; j++){
            int t;
            cin >> t;
            if(i<j && t>=0){
                we[k].w = t;
                we[k].a = i; we[k].b = j;
                k++;
            }
            //cout << t<<":"<<i<<","<<j<<"\t";
        }
        //cout << endl;
    }
    //cout << "k"<<k<<endl;
}
signed main(){    
    while(cin >> n){
        get();
        cout << solve() <<  endl;
    }
}

C++: #define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES
//#include "MyMath.h"
//#include "MyDisjointset.h"
#include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <functional>
#include <stdio.h>
using namespace std;
typedef pair<int, int> P;
const int INF = 2000000000;
class Disjointset {
public:
	vector<int> rank, p;
	Disjointset(int n);
	int find(int x);
	bool same(int x, int y);
	void unite(int x, int y);
};

Disjointset::Disjointset(int n) {
	rank.resize(n, 0);
	for (int i = 0; i < n; i++)
		p.push_back(i);
}
int Disjointset::find(int x) {
	if (x != p[x])
		return p[x] = find(p[x]);
	else return x;
}
bool Disjointset::same(int x, int y) {
	return find(x) == find(y);
}
void Disjointset::unite(int x, int y) {
	int px = find(x);
	int py = find(y);
	if (rank[px] > rank[py]) {
		p[py] = px;
	}
	else {
		p[px] = py;
		if (rank[px] == rank[py]) rank[py]++;
	}
}
struct edge {
	int from, to, cost;
	bool operator < (const edge &e) const {
		return cost > e.cost;
	}
};
int main() {
	int n; cin >> n;
	priority_queue <edge> q;
	for (int i = 0; i < n; i++)for (int j = 0; j < n; j++) {
		int c; cin >> c;
		if (c != -1) { edge e; e.from = i; e.to = j; e.cost = c; q.push(e); }
	}
	Disjointset D(n);
	int res = 0;
	while (!q.empty()) {
		edge e = q.top(); q.pop();
		int f = e.from, t = e.to, c = e.cost;
		if (D.same(f, t)) continue;
		res += c;
		D.unite(f, t);
	}
	cout << res << endl;
	return 0;
}