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 <bits/stdc++.h>

using namespace std;
using LL = long long;
using P = pair<int, int>;
using Tapris = tuple<int, int, int>;

#define REP(i, n) for(LL i = 0; i < n; ++i)
#define FOR(i, a, n) for(LL i = a; i < n; ++i)
#define pb(a) push_back(a)
#define all(x) (x).begin(),(x).end()

const int INF = (int)1e9;
const LL INFL = (LL)1e15;
const int MOD = 1e9 + 7;

int dy[]={0, 0, 1, -1, 0};
int dx[]={1, -1, 0, 0, 0};

//#define int long long

class UnionFind {
    private:
        vector<int> parent;  // 親
        vector<int> rank;    // 木の深さ
    public:
        UnionFind(int n);          // n要素で初期化
        int Find(int x);           // 木の根を返す
        void Unite(int x, int y);  // xとyの属する集合を併合
        bool Same(int x, int y);   // xとyが同じ集合に属するか否か
};

UnionFind::UnionFind(int n) : parent(vector<int>(n)), rank(vector<int>(n)) {
    for (int i = 0; i < n; i++) {
        parent[i] = i;
        rank[i] = 0;
    }
}

int UnionFind::Find(int x) {
    if (parent[x] == x) {
        return x;
    } else {
        return parent[x] = Find(parent[x]);
    }
}

void UnionFind::Unite(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]++;
    }
}

bool UnionFind::Same(int x, int y) { return Find(x) == Find(y); }

template <typename T>
struct Edge {
    int src, to;
    T cost;
    Edge(int to, T cost) : src(-1), to(to), cost(cost) {}
    Edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}
};

template<typename T>
using Edges = vector<Edge<T>>;
template<typename T>
using WeightedGraph = vector<Edges<T>>;
using UnWeightedGraph = vector<vector<int>>;
template<typename T>
using Matrix = vector<vector<T>>;


template <typename T>
class Kruskal {
    private:
        int V; // 頂点数
        Edges<T> edges;
        WeightedGraph<T> minimumSpanningTree;
    public:
        Kruskal(int n);
        void AddEdge(int from, int to, T cost);
        T Run();
        WeightedGraph<T> GetMinimumSpanningTree();
};

template <typename T>
Kruskal<T>::Kruskal(int n) : V(n), minimumSpanningTree(n) {}

template <typename T>
void Kruskal<T>::AddEdge(int from, int to, T cost) {
    edges.push_back(Edge<T>(from, to, cost));
}

template <typename T>
T Kruskal<T>::Run() {
    sort(edges.begin(), edges.end(), [](const Edge<T> &a, const Edge<T> &b){
        return a.cost < b.cost;
    });
    UnionFind tree(V);
    T total = 0;
    
    for(auto &e : edges){
        if(!tree.Same(e.src, e.to)){
            tree.Unite(e.src, e.to);
            total += e.cost;
            minimumSpanningTree[e.src].push_back(Edge<T>(e.to, e.cost));
            minimumSpanningTree[e.to].push_back(Edge<T>(e.src, e.cost));
        }
    }

    return total;
}

template <typename T>
WeightedGraph<T> Kruskal<T>::GetMinimumSpanningTree(){
    return minimumSpanningTree;
}

/*************** using variables ***************/

/**********************************************/

signed main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int n;
    cin >> n;
    Kruskal<int> kruskal(n);
    REP(i, n){
        REP(j, n){
            int x;
            cin >> x;
            if(x != -1) kruskal.AddEdge(i, j, x);
        }
    }
    int total = kruskal.Run();
    cout << total << endl;
}

C++: # if 0
2次元配列の扱いの取り扱いに慣れておらず、2時間ほど時間がかかった。
# endif
#include <iostream>
#include <algorithm>
using namespace std;
int P[10000];
void init(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);
}

int main()
{
  int N;
  cin >> N;
  int A[N][N];
  for (int i = 0; i < N; i++) {
    for (int j = 0; j < N; j++) {
      int x;
      cin >> x;
      A[j][i] = x;
    }
  }
  int cost[10000], src[10000], dst[10000];
  int M = 0;
  for (int r = 0; r < N; r++)
  {
    for (int c = r + 1; c < N; c++)
    {
      if (A[r][c] > -1)
      {
        cost[M] = A[r][c];
        src[M] = r;
        dst[M] = c;
        M += 1;
      }
    }
  }
  init(N);
  pair<int, pair<int, int> > edges[M + 1];
  for (int m = 0; m < M; m++)
  {
    edges[m] = make_pair(cost[m], make_pair(src[m], dst[m]));
  }
  sort(edges, edges+M);
  int sum = 0;
  for (int i=0; i<M; i++) {
    if (is_same_set(edges[i].second.first, edges[i].second.second))
      continue;
    unite(edges[i].second.first, edges[i].second.second);
    sum += edges[i].first;
  }
  cout << sum << endl;
}