Go: package main

import (
	"bufio"
	"os"
	"strconv"
	"fmt"
)

const (
	MAX = 100
	INF = 1 << 21
	WHITE = 0
	GRAY = 1
	BLACK = 2
)

var (
	n int
	M [MAX][MAX]int
)

func prim() int {
	var u, minv int
	var d [MAX]int
	var p [MAX]int
	var color [MAX]int

	for i := 0; i < n; i++ {
		d[i] = INF
		p[i] = -1
		color[i] = WHITE
	}
	d[0] = 0
	for {
		minv = INF
		u = -1
		for 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 v := 0; v < n; v++ {
			if color[v] != BLACK && M[u][v] != INF {
				if d[v] > M[u][v] {
					d[v] = M[u][v]
					p[v] = u
					color[v] = GRAY
				}
			}
		}
	}
	sum := 0
	for i := 0; i < n; i++ {
		if p[i] != -1 {
			sum += M[i][p[i]]
		}
	}
	return sum
}

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	scanner.Split(bufio.ScanWords)
	scanner.Scan()
	n, _ = strconv.Atoi(scanner.Text())
	for i := 0; i < n; i++ {
		for j := 0; j < n; j++ {
			scanner.Scan()
			e, _ := strconv.Atoi(scanner.Text())
			if e == -1 {
				M[i][j] = INF
			} else {
				M[i][j] = e
			}
		}
	}
	fmt.Println(prim())
}

C++: ////////////////////////////////////////
//ALDS1_12_A:   Minimum Spanning Tree
//????????§??????????????±??£?????????????§£(?´?????????????????§????)??????????????¢??°?????£????????????????????????
////////////////////////////////////////

#include <iostream>
#include <algorithm> // next_permutation
#include <iomanip>
#include <cmath>
#include <vector>
#include <sstream>
#include <string>
#include <cstdio>
#include <stack>
#include <queue>
#include <list>
#include <numeric> //accumulate
//#include <unordered_map> //???????????\??¢??°
#include <fstream> //ifstream, ofstream

//#define NDEBUG //NDEBUG???#include <cassert>???????????????????????????????????´???assert?????????????????????????????????NDEBUG?????????????????????????????????????????????
#include <cassert> //assert


using namespace std;

//???????????°??????TEST????????????????????¬???????????????????????????????????¢????????????
#define TEST //*******************************************************************************************************************************************
//?????????????????¨??§?????\????????????????????????????????°?????????????¨???????????????????dout???????????????????????§???????????¬??????????????????????????????????????????
//??????????????????????????????????????????????????????????§?CPU???????£??????????????????§???TLE?????????????????????????????????????????§????????¨???                           ????????????????????????cerr????????£????????????????????????????????????
#ifdef TEST
#define dout cout
#define din cin
#else
stringstream dummy; //???????????°??????dout?????????????????????????????????
#define dout dummy.str(""); dummy.clear(stringstream::goodbit); dummy //dummy?????????????????????????????????????????¨?????¢?????¢???????????????
//???????????¨??????????????????goodbit?????????????????????????????¨???????????´????????????????????????????????¨?????°?????????????????§???????????????????????????????????? http://d.hatena.ne.jp/linden/20060427/p1
#endif

//?¨??????????????????????????????????????????????????????´??????OUTPUTFILE????????????????????¬???????????????????????????????????¢????????????
//#define OUTPUTFILE "output.txt" //*******************************************************************************************************************************************
#ifdef OUTPUTFILE
#define dout outputfile //??¨????????????TLE????????§?????????dout?????¨??¨??????????????¢????????????????????????????????????????????§????¨????????????´??????????????£??????????????????????¨?????????????????????????????????????????????§?????????????????????
ofstream outputfile(OUTPUTFILE);
#define OutputFilePath "/Users/Nag/Documents/Prgm/Test/DerivedData/Test/Build/Products/Debug/output.txt"
#endif

//?¨??????\???????????????????????????????????????????????´??????INPUTFROMTEXTFILE????????????????????¬???????????????????????????????????¢????????????
//#define INPUTFILE "input.txt" //*******************************************************************************************************************************************
#ifdef INPUTFILE
#define din inputfile
ifstream inputfile(INPUTFILE);
#endif


#define disp(A) dout << #A << " = " << setw(3) << (A) << endl
#define disP(A) dout << setw(3) << (A) << " " // << setw(3) ??????????????\????????????
#define rep(i,a,n) for(int (i)=(a); (i)<(n); (i)++)
#define dispAll(A,n) dout << #A << " = "; rep(j, 0, (n)) {disP(A[j]);} dout << endl

typedef pair<int,int> pii;
typedef vector<int> vi;
typedef unsigned long long ll;


const int INF = 2e9-1;

#define N_MAX 100 //num of vertex
//#define M_MAX 10000 //mum of edge

int N;
int matrix[N_MAX][N_MAX];

vector< pair< int, pii > > edge;


int color[N_MAX]; //????????¨???????±?????????????vertex???????????°?????§???????????????????´?????????????????§????
#define isSame(x,y) (find(x)==find(y))
#define unite(x,y) color[find(x)] = find(y)
int find(int x) {
    if( color[x]==x ) return x;
    return color[x] = find( color[x] );
}





int main() {
    
    //read input data
    din >> N;
    rep(i,0,N) {
        rep(j,0,N) {
            din >> matrix[i][j];
        }
    }
    
    //construct edge data
    //matrix???????????????????????????????????????
    rep(i,0,N) {
        rep(j,i+1,N) {
            if( matrix[i][j] != -1)
                edge.push_back(make_pair(matrix[i][j], make_pair(i, j)));
        }
    }
    
    //??????????????±???????????????????????§??????????????????
    sort(edge.begin(), edge.end());
    
    
    //???vertex????????????????????????
    rep(i,0,N) color[i] = i;
    
    
    //??????????????????
    ll minCost = 0;
    while( !edge.empty() ) {
//        dout << "----------------------\n";
        
        pair< int, pii > e = edge.front(); //?????????????°???????e?????????
        
        int cost = e.first;
        int v1 = e.second.first; //???e?????????v1??¨v2?????\?¶?????????????
        int v2 = e.second.second;
        
//        //test display
//        dout << "cost=" << cost << " : " << v1 << "--" << v2 << endl;
//        disp(isSame(v1, v2));
        
        
        if( !isSame(v1, v2) ) { //v1??¨v2???????????\?????°????????????????±???????????????°??????e????????¨??????v1??¨v2?????\?¶???????????????§???????????°??????????????£?????´?????????e??????????????¨???????????§????????????????????§NG
//            dout << "?????????????????¨!!\n";
            
            minCost += cost;
            unite(v1, v2);
        }
        
//        //test display
//        rep(i,0,N) {
//            disP(i); disp(color[i]);
//        }
//        disp(minCost);
        
        
            
        edge.erase(edge.begin());
    }
    
    
    
    //output
    dout << minCost << endl;
   
    
    
    
#ifdef INPUTFILE
    inputfile.close();
#endif
    
#ifdef OUTPUTFILE
    outputfile.close();
    cout << "\"" << OutputFilePath << "\"" << endl;
#endif
    
    return 0;
}

C++: #include<bits/stdc++.h>
#define inf 10000000
using namespace std;
int main(){
  int n,d[101][101],a[101],ans=0,c[101]={};
  cin>>n;
  for(int i=0;i<n;i++)a[i]=inf;
  for(int i=0;i<n;i++)
    for(int j=0;j<n;j++){
      cin>>d[i][j];
      if(d[i][j]==-1)d[i][j]=inf;    
    }
  a[0]=0;
  int s=0;
  while(1){
    c[s]=1;
    ans+=a[s];
    int mn=inf,ns=-1;
    for(int i=0;i<n;i++)
      a[i]=min(a[i],d[s][i]);

    for(int i=0;i<n;i++)
      if(!c[i]&&a[i]<mn)ns=i,mn=a[i];

    if(ns==-1)break;
    s=ns;
  }
  cout<<ans<<endl;
  return 0;
}