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>
#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;
      // cout << "cost: " << x << " = " <<  j << "->"  << i << endl;
    }
  }
  // cout << "-----エッジ-------" << endl;
  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;
        // cout << "cost: " << cost[M] << " = " <<  src[M] << "->"  << dst[M] << endl;
        M += 1;
      }
    }
  }
  // cout << "M = " << M << " N = " << N << endl;
  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]));
  }
  // cout << "-----sort前-------" << endl;
  // for (int i = 0; i < M; i++)
  //     cout << "cost: " << edges[i].first << " = " << edges[i].second.first << "->"  << edges[i].second.second << endl;
  sort(edges, edges+M);
  // cout << "-----sort後-------" << endl;
  // for (int i = 0; i < M; i++)
  //     cout << "cost: " << edges[i].first << " = " << edges[i].second.first << "->"  << edges[i].second.second << endl;
  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;
}

C++: // http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ALDS1_12_A
// Graph II - Minimum Spanning Tree
// TODO: REVIEW

#define SUBMIT

#include <utility>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <vector>
#include <limits>
#include <numeric>
#include <functional>
#include <set>
#include <cmath>
#include <cstring>
#include <climits>
#include <memory>

using namespace std;
using ui64 = unsigned long long;
using i64 = long long;

enum class color_type {
    white,
    gray,
    black
};

const int INF = INT_MAX;
const ui64 MAX_N = 100;
int adj[MAX_N][MAX_N];
int n;

int prim() {
    int d[MAX_N], p[MAX_N];
    color_type color[MAX_N];

    for (int i = 0; i < n; ++i) {
        d[i] = INF;
        p[i] = -1;
        color[i] = color_type::white;
    }

    d[0] = 0;

    while (true) {
        // 未訪問のノード
        int u = -1, min_v = INF;
        for (int i = 0; i < n; ++i) {
            if (d[i] < min_v && color[i] != color_type::black) {
                u = i;
                min_v = d[i];
            }
        }

        if (u == -1) break;

        color[u] = color_type::black;
         for (int v = 0; v < n; ++v) {
            if (color[v] != color_type::black && adj[u][v] != INF) {
                if (adj[u][v] < d[v]) {
                    d[v] = adj[u][v];
                    p[v] = u;
                    color[v] = color_type::gray;
                }
            }
        }
    }

    int sum = 0;
    for (int i = 0; i < n; ++i) {
        if (p[i] != -1) sum += adj[i][p[i]];
    }

    return sum;
}

int main() {
#ifdef SUBMIT
    auto& stream = cin;
#else
    string program_input_str = R"(20
 -1 100 2 -1 5 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
 100 -1 3 1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
 2 3 -1 1 10 12 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
 -1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
 5 -1 10 -1 -1 2 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
 -1 -1 12 -1 2 -1 19 3 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
 -1 -1 -1 -1 -1 19 -1 17 -1 -1 3 4 -1 -1 -1 -1 -1 -1 -1 -1
 -1 -1 -1 -1 -1 3 17 -1 5 2 1000 -1 -1 -1 -1 -1 -1 -1 -1 -1
 -1 -1 -1 -1 -1 -1 -1 5 -1 5 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
 -1 -1 -1 -1 -1 -1 -1 2 5 -1 1 -1 -1 50 -1 -1 -1 -1 -1 -1
 -1 -1 -1 -1 -1 -1 3 1000 -1 1 -1 8 100 -1 -1 -1 -1 -1 -1 -1
 -1 -1 -1 -1 -1 -1 4 -1 -1 -1 8 -1 7 -1 -1 -1 -1 -1 -1 -1
 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 100 7 -1 1 5 2 -1 -1 -1 -1
 -1 -1 -1 -1 -1 -1 -1 -1 -1 50 -1 -1 1 -1 -1 -1 -1 -1 -1 -1
 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 5 -1 -1 2 3 -1 3 -1
 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 2 -1 2 -1 -1 -1 -1 -1
 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 3 -1 -1 5 -1 -1
 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 5 -1 1 1
 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 3 -1 -1 1 -1 3
 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 1 3 -1
)";
    stringstream stream(program_input_str);
#endif
    memset(adj, 0, sizeof(adj));
    stream >> n;

    int w;
    for (int y = 0; y < n; ++y) {
        for (int x = 0; x < n; ++x) {
            stream >> w;
            if (w == -1) adj[y][x] = INF;
            else adj[y][x] = w;
        }
    }

    cout << prim() << endl;
    return 0;
}