Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

type ElementKey int
type Node struct {
	key    ElementKey
	parent *Node
	rank   int
}

func newNode(key ElementKey) *Node {
	node := Node{}
	node.key = key
	node.parent = &node
	node.rank = 0

	return &node
}

func union(x, y *Node) {
	xSet := findSet(x)
	ySet := findSet(y)
	link(xSet, ySet)
}

func link(x, y *Node) {
	if x.rank > y.rank {
		y.parent = x
	} else {
		x.parent = y

		if x.rank == y.rank {
			y.rank++
		}
	}
}

func findSet(x *Node) *Node {
	if x != x.parent {
		x.parent = findSet(x.parent)
	}
	return x.parent
}

func newDisjointSets(nSets int) *DisjointSets {
	sets := DisjointSets{}
	sets.keyToNode = map[ElementKey]*Node{}
	for i := 0; i < nSets; i++ {
		key := ElementKey(i)
		sets.keyToNode[key] = newNode(key)
	}
	return &sets
}

type DisjointSets struct {
	keyToNode map[ElementKey]*Node
}

func (sets *DisjointSets) union(x, y ElementKey) {
	xNode := sets.keyToNode[x]
	yNode := sets.keyToNode[y]
	union(xNode, yNode)
}

func (sets DisjointSets) findSet(x ElementKey) *Node {
	xNode := sets.keyToNode[x]
	return findSet(xNode)
}

type Edge struct {
	i, j   int
	weight int
}

func main() {
	scanner.Split(bufio.ScanWords)

	n := nextInt()
	sets := newDisjointSets(n)
	edges := []Edge{}
	a := make([][]int, n)
	for i := 0; i < n; i++ {
		a[i] = make([]int, n)
		for j := 0; j < n; j++ {
			a[i][j] = nextInt()
			if a[i][j] >= 0 {
				e := Edge{
					i:      i,
					j:      j,
					weight: a[i][j],
				}
				edges = append(edges, e)
			}
		}
	}
	sort.Slice(edges, func(i, j int) bool { return edges[i].weight < edges[j].weight })
	totalWeight := 0
	for _, edge := range edges {
		i := ElementKey(edge.i)
		j := ElementKey(edge.j)
		if sets.findSet(i) != sets.findSet(j) {
			totalWeight += edge.weight
			sets.union(i, j)
		}
	}
	fmt.Println(totalWeight)
}

C++: #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 long long ll;


const int INF = 2e9-1;
const ll INFLL = 1e18-999;

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

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

ll minCostTo[N_MAX]; //?????¨?§??????????MST????????????i??????????°???????????????¨???????????????Daijkstra?????§??´??°????????????
vi vertices; //??????MST????????????????????????????????¨???






int main() {
    
    //read input data and initialize
    din >> N;
    int cost;
    rep(i,0,N) {
        vertices.push_back(i); //??¨????????????????????????
        minCostTo[i] = INFLL; //??¨????????????????????????????????§??§?????????
        
        rep(j,0,N) {
            din >> cost;
            matrix[i][j] = (cost==-1 ? INF : cost); //??????????????¨??????????????????INF??¨??????
        }
    }
    
//    //test display
//    rep(i,0,N) {
//        rep(j,0,N) {
//            dout << matrix[i][j] << " ";
//        }
//        dout << endl;
//    }
//    dout << endl;

    
    
    
    //????????????
    ll ans = 0;
    int x; //????????????????????????MST?????????????????????
    minCostTo[0] = 0; //x=0???????§????????????????0??§?????????
    while( !vertices.empty() ) {
//        dout << "---------------------\n";
        
        //?????????????????????????????????????????°????????????????°??????????x?????¢?´¢
        x = vertices.front();
        vi::iterator itr_to_x = vertices.begin();
        for( vi::iterator itr=vertices.begin(); itr!=vertices.end(); itr++) {
            if( minCostTo[x] > minCostTo[*itr] ) {
                x = *itr;
                itr_to_x = itr;
            }
        }
        
//        dout << "add vertex " << x << endl;
        
        //MST?????????x??????????????????????????????????????????????????????x????????????????????§???????°?????????´??°??§?????????????????§??????
        vertices.erase(itr_to_x);
        ans += minCostTo[x];
        minCostTo[x] = 0; //MST????????????????????????????????????0?????????
        
        for( vi::iterator itr=vertices.begin(); itr!=vertices.end(); itr++) {
            if( minCostTo[*itr] > minCostTo[x] + matrix[x][*itr] ) {
                minCostTo[*itr] = minCostTo[x] + matrix[x][*itr];
            }
        }
        
//        //test display
//        dispAll(vertices, vertices.size());
//        
//        rep(i,0,N) {
//            disP(i); disp(minCostTo[i]);
//        }
//        
//        disp(ans);
    }
    
    
    
//    output
    dout << ans << endl;
    
    
    
#ifdef INPUTFILE
    inputfile.close();
#endif
    
#ifdef OUTPUTFILE
    outputfile.close();
    cout << "\"" << OutputFilePath << "\"" << endl;
#endif
    
    return 0;
}