Go: package main

import (
	"bufio"
	"fmt"
	"log"
	"math"
	"os"
	"strconv"
	"strings"
)

type color int

const (
	white color = iota // 未訪問
	mst                // MSTとして確定
)

// 頂点
type node struct {
	matrix   []int // 隣接行列
	minToMst int   // MSTへの重み最小値
	parent   int   // 最小値の相手
	color    color
}

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	n := eGetInt(scanner)

	nodes := make([]node, n)
	for i := 0; i < n; i++ {
		nodes[i].matrix = eAtoi((strings.Fields(eGetline(scanner))))
		nodes[i].parent = -1
		nodes[i].minToMst = math.MaxInt16
		nodes[i].color = white
	}

	// 初回はnode[0]がMSTに入るように,minToMstを設定
	nodes[0].minToMst = 0
	for { // 全部チェックするまで

		// どの頂点を新たにMSTに取り込むか
		min := math.MaxInt16
		u := -1
		for i := 0; i < n; i++ {
			if nodes[i].color == mst {
				continue
			}
			if nodes[i].minToMst < min {
				u = i
				min = nodes[i].minToMst
			}
		}

		if u == -1 {
			break
		}
		// MSTに追加する頂点決定
		nodes[u].color = mst

		// MSTに属していない頂点それぞれについて,MSTとの最小の重みを更新
		for i := 0; i < n; i++ {
			if nodes[i].color == mst {
				continue
			}
			if nodes[i].matrix[u] == -1 {
				// 接してない
				continue
			}
			if nodes[i].matrix[u] < nodes[i].minToMst {
				// node[u]との重みが最小
				nodes[i].minToMst = nodes[i].matrix[u]
				nodes[i].parent = u
			}
		}
	}

	sum := 0
	for _, v := range nodes {
		// node[0]はparentが入っていない
		if v.parent != -1 {
			sum += v.matrix[v.parent]
		}
	}
	fmt.Println(sum)
}

func eGetInt(scanner *bufio.Scanner) int {
	t := eAtoi(strings.Fields(eGetline(scanner)))
	return t[0]
}

func eGetline(p *bufio.Scanner) string {
	if !p.Scan() {
		if err := p.Err(); err != nil {
			log.Fatal(err.Error())
		}
		return ""
	}
	return p.Text()
}

func eAtoi(s []string) []int {
	ints := make([]int, len(s))
	var err error
	for i, e := range s {
		ints[i], err = strconv.Atoi(e)
		if err != nil {
			log.Fatal(err.Error())
		}
	}
	return ints
}

C++: #include <bits/stdc++.h>
using namespace std;

const int INFY = numeric_limits<int>().max();
int n;
typedef vector<vector<int>> GM;

int prim(GM &G) {
    vector<bool> inmst(n, false);
    vector<int> d(n, INFY);
    vector<int> p(n, -1);

    d[0] = 0;
    while(true) {
        int u = -1, minv = INFY;
        for (int i = 0; i < n; ++i) {
            if(!inmst[i] && d[i] < minv) {
                u = i;
                minv = d[i];
            }
        }
        if (u == -1) break;
        inmst[u] = true;
        for (int i = 0; i < n; ++i) {
            if (!inmst[i] && G[u][i] != INFY) {
                if (G[u][i] < d[i]) {
                    d[i] = G[u][i];
                    p[i] = u;
                }
            }
        }
    }
    int sum = 0;
    for (int i = 0; i < n; ++i) {
        if(p[i] != -1) sum += G[i][p[i]];
    }
    return sum;
}

int main() {
    cin >> n;
    GM G(n, vector<int>(n));
    for(int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            int c;
            cin >> c;
            G[i][j] = c == -1 ? INFY : c;
        }
    }
    cout << prim(G) << endl;
}

C++: #include <iostream>
using namespace std;
static const int MAX = 100;
static const int INFTY = (1 << 21);
static const int NOT_VISIT = 0;
static const int VISITING = 1;
static const int VISITED = 2;
int n;
int M[MAX][MAX];
int prim()
{
    int minv, u;
    int isVisited[MAX], d[MAX], p[MAX];
    for (int i = 0; i < n; i++) {
	isVisited[i] = NOT_VISIT;
	d[i] = INFTY;
	p[i] = -1;
    }
    d[0] = 0;
    while (1) {
	minv = INFTY;
	u = -1;
	for (int i = 0; i < n; i++) {
	    if (minv > d[i] && isVisited[i] != VISITED) {
		u = i;
		minv = d[i];
	    }
	}
	if (u == -1) {
	    break;
	}

	isVisited[u] = VISITED;
	for (int j = 0; j < n; j++) {
	    if (isVisited[j] != VISITED && M[u][j] != INFTY) {
		if (d[j] > M[u][j]) {
		    d[j] = M[u][j];
		    p[j] = u;
		    isVisited[j] = VISITING;
		}
	    }
	}
    }
    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>
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];

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

  d[0] = 0;

  while (1) {
    minv = INFTY;
    u = -1;
    for (int i = 0; i < n; i++) {
      if (minv > d[i] && color[i] != BLACK){
        u = i;
        minv = d[i];
      }
    }
    if (u == -1) break;
    color[u] = BLACK;
    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;
        }
      }
    }
  }
  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;

  return 0;
}