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;
typedef long long ll;
#define rep(i, start, n) for (int i = (int)(start); i < (int)(n); ++i)
static const int MAX = 100;
static const int INFTY = (1 << 30);
static const int WHITE = 0;
static const int GRAY = 1;
static const int BLACK = 2;
int n, M[MAX][MAX]; //nは要素数、Mは隣接行列
int prim()
{
    int u, minv;                    //uは最短距離の相手、minvは最短距離
    int d[MAX], p[MAX], color[MAX]; //dは始点からの距離、pは親,colorは訪問状態
    rep(i, 0, n)                    //初期設定
    {
        d[i] = INFTY;
        p[i] = -1;
        color[i] = WHITE; //未訪問
    }
    d[0] = 0; //始点のdは0
    while (1)
    {
        minv = INFTY;
        u = -1;
        rep(i, 0, n) //未完了の点のうち始点からの最短距離は？
        {
            if (minv > d[i] && color[i] != BLACK) //未完了の点がある
            {
                u = i;
                minv = d[i];
            }
        }
        if (u == -1)
            break;
        color[u] = BLACK;
        rep(i, 0, n)
        {
            if (color[i] != BLACK && M[u][i] != INFTY)
            {
                if (d[i] > M[u][i])
                {
                    d[i] = M[u][i];
                    p[i] = u;
                    color[i] = GRAY;
                }
            }
        }
    }
    int sum = 0;
    rep(i, 0, n)
    {
        if (p[i] != -1)
            sum += M[i][p[i]];
    }
    return sum;
}
int main()
{
    cin >> n;
    rep(i, 0, n)
    {
        rep(j, 0, n)
        {
            int e;
            cin >> e;
            M[i][j] = (e == -1) ? INFTY : e;
        }
    }
    cout << prim() << endl;
}

C++: 

#include <iostream>
using namespace std;
#define INFTY  (1 << 30)//极大值
#define NIL  -1
const int TOTAL = 101;
int totalnode,color[TOTAL], matrix[TOTAL][TOTAL], DistanceBetweenParent[TOTAL], parent[TOTAL];//颜色0白色，1为灰色，2是黑色
void includeMinimumSpanningTree()
{
	int u,min;
	DistanceBetweenParent[0] = 0;//这个点到其父节点的距离为0
	parent[0] = -1;//根的父节点为-1
	while (1)
	{
		min = INFTY;
		for (int i = 0; i < totalnode; i++)//找出非黑色同时到父节点加权值最少的节点
			if (color[i] != 2 && DistanceBetweenParent[i] < min)//第一次只有根
				min = DistanceBetweenParent[i], u = i;//以u寻找连接它的其他点
		color[u] = 2;//u的颜色为黑色
		if (min == INFTY)break;//寻找结束
		for (int i = 0; i < totalnode; i++)//寻找相连的点
		{
			if (color[i] != 2 && matrix[u][i]!= -1)//未发现且存在边
			{
				if (matrix[u][i] < DistanceBetweenParent[i])//当前点连接父节点的加权值大于新的加权值
				{
					color[i] = 1;//灰色
					DistanceBetweenParent[i] = matrix[u][i];
					parent[i] = u;
				}
			}
		}
	}
}
int main()
{	
	int inputdata,minmum=0;
	cin >> totalnode;
	for (int i = 0; i < totalnode; i++)color[i] = 0;//颜色初始化
	for(int i = 0; i < totalnode; i++)DistanceBetweenParent[i] = INFTY;//将距离初始化为最大值
	for (int i = 0; i < totalnode; i++)//顶点i 到顶点j的距离，不存在的为-1；
		for (int j = 0; j < totalnode; j++)
			cin >> inputdata, matrix[i][j] = inputdata;
	includeMinimumSpanningTree();
	for (int i = 0; i < totalnode; i++)
		minmum += DistanceBetweenParent[i];
	cout << minmum << endl;

}

C++: 
#include<stdio.h>

#define MAX 100
#define INFINITY 10000

int n;
int M[MAX][MAX], color[MAX], d[MAX], p[MAX];

enum {
    WHITE,
    GRAY,
    BLACK
};

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

    while (true) {
        int min = INFINITY;
        int target = -1;
        for (int i = 0; i < n; ++i) {
            if (d[i] < min && color[i] != BLACK) {
                min = d[i];
                target = i;
            }
        }
        if (target == -1) break;

        color[target] = BLACK;

        for (int v = 0; v < n; ++v) {
            if (color[v] != BLACK && M[target][v] > 0) {
                if(M[target][v] < d[v]) {
                    d[v] = M[target][v];
                    p[v] = target;
                    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() {
    scanf("%d", &n);
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            scanf("%d", &M[i][j]);
        }
    }
    printf("%d\n", prim());
    return 0;
}