SDA - 2019 2020 - test5 - task 2

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <unordered_map>
#include <unordered_set>
#include <queue>
#include <set>

using namespace std;

unordered_map<int, unordered_set<int>> graph;
set<int> result;
bool visited[100000] {false};

void bfs(int start, int distance){
    queue<pair<int, int>> next;
    next.push({start, 0});
    visited[start] = true;
    while(!next.empty()){
        auto curr = next.front();
        next.pop();
        auto neighbours = graph[curr.first];
        if(curr.second == distance - 1) {
            for(auto i : neighbours) {
                if(!visited[i]){
                    result.insert(i);
                }
            }
        }
        else {
            for(auto i : neighbours) {
                if(!visited[i]){
                    next.push({i, curr.second + 1});
                    visited[i] = true;
                }
            }
        }
    }
}

int main() {
    int n, m, k, a, b;
    cin >> n >> m >> k;

    for(int i = 0; i < m; i++){
        cin >> a >> b;
        graph[a].insert(b);
        graph[b].insert(a);
    }
    if(k == 0){
        cout << n;
        return 0;
    }

    bfs(n, k);

    if(result.empty()){
        cout << -1;
        return 0;
    }

    for(int i : result)
        cout << i << " ";
    return 0;
}