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 <bits/stdc++.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 u, minv;
    int d[MAX], p[MAX], color[MAX];

    // init
    for (int i = 0; i < n; i++) {
        d[i] = INFTY; // ノードごとの最小エッジスコア
        p[i] = -1; // Minimum spanning treeでの親
        color[i] = WHITE;
    }

    d[0] = 0; // node 0からスタート

    while (true) {
        minv = INFTY;
        u = -1;

        // MSTに入っていない点の中で、最小エッジスコアが最小のものを選択 -> u
        for (int i = 0; i < n; i++) {
            if (minv > d[i] && color[i] != BLACK) {
                u = i;
                minv = d[i];
            }
        }

        // 行ける範囲にいった場合終わり
        if (u == -1)
            break;
        
        // uに到達
        color[u] = BLACK;

        // uの近傍の最小エッジスコアを更新
        for (int v = 0; v < n; v++) {
            if (color[v] != BLACK && M[u][v] != INFTY) {
                if ( d[v] > M[u][v] ) {
                    d[v] = M[u][v];
                    p[v] = u;
                    color[v] = GRAY;
                }
            }
        }
    }

    // back pointerから重みの和を計算
    int sum = 0;
    for (int i = 0; i < n; i++) {
        if ( p[i] != -1 )
            sum += M[i][p[i]];
    }
    return sum;
}

int main() {
    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;
}

C++: #include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <utility>

using namespace std;

#define REP(var, count) for(int var=0; var<count; var++)

struct unionf {
	
	vector<int> data;
	
	unionf(int n) {
		REP(i, n+1) { data.push_back(i); }
	}
	
	int root(int a) {
		if(data[a] == a) { return a; }
		return (data[a] = root(data[a]));
	}
	
	bool issame(int a, int b) {
		return root(a) == root(b);
	}
	
	void unite(int a, int b) {
		data[root(a)] = root(b);
	}

};

int main(void) {
	int N;
	cin >> N;
	
	vector<pair<int, pair<int, int> > > edge;
	
	REP(x, N) {
		REP(y, N) {
			int c;
			cin >> c;
			if ( x <= y ) { continue; }
			if ( c == -1 ) { continue; }
			edge.push_back( make_pair(c, make_pair(x,y)) );
		}
	}
	
	sort(edge.begin(), edge.end());
	
	unionf uf(N);
	
	int cnt = 0;
	int ret = 0;
	REP(i, edge.size() ) {
		
		pair<int, int> sd = edge[i].second;
		if ( ! uf.issame( sd.first, sd.second ) ) {
			cnt ++;
			ret += edge[i].first;
		}
		
		if ( cnt == N-1 ) { break; }
	}
	
	cout << ret << endl;
}

C++: #include<iostream>
using namespace std;
#define S 100
#define INF 2001
#define WHITE 0;
#define GRAY 1;
#define BLACK 2
int n,a[S][S];

int prim(){
    int u,mina;
    int d[S],p[S],cnd[S];

    for(int i=0;i<n;i++){
        d[i]=INF;
        p[i]=-1;
        cnd[i]=WHITE;
    }

    d[0]=0;
    while(1){
        mina=INF;
        u=-1;
        for(int i=0;i<n;i++){
            if(mina>d[i]&&cnd[i]!=BLACK){
                u=i;
                mina=d[i];
            }
        }
        if(u==-1)break;
        cnd[u]=BLACK;
        for(int v=0;v<n;v++){
            if(cnd[v]!=BLACK&&a[u][v]!=INF){
                if(d[v]>a[u][v]){
                    d[v]=a[u][v];
                    p[v]=u;
                    cnd[v]=GRAY;
                }
            }
        }
    }
            int sum=0;
        for(int i=0;i<n;i++){
            if(p[i]!=-1)sum+=a[i][p[i]];
        }
    return sum;
}

int main(){
    cin>>n;
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++){
            int e;
            cin>>e;
            if(e==-1)a[i][j]=INF;
            else a[i][j]=e;
        }
    }
    cout<<prim()<<endl;
    return 0;

}