BFS

#define _CRT_SECURE_NO_WARNINGS
#include <cstdio>
#include <iostream>
#include <cmath>
#include <vector>
#include <queue>
#include <algorithm>
using namespace std;
vector<vector <int>> way;
queue<int> q;
vector<bool> used;
vector<int> ans;
void bfs(int pos)
{
    q.pop();
    for (int i = 0; i < way[pos].size(); i++)
        if (used[way[pos][i]]) {
            used[way[pos][i]] = false;
            q.push(way[pos][i]);
            ans.push_back(way[pos][i]);
        }
    if (!q.empty())
        dfs(q.front());
    return;
}

int main()
{
    int n, s;
    scanf("%d %d", &n, &s);
    way.resize(n + 1);
    used.resize(n + 1, true);
    ans.push_back(s);
    used[s] = false;
    q.push(s);
    for (int i = 0; i < n - 1; i++)
    {
        int a, b;
        scanf("%d %d", &a, &b);
        way[a].push_back(b);
        way[b].push_back(a);
    }
    bfs(s);

    for (int i = 0; i < ans.size(); i++)
        printf("%d ", ans[i]);

    return 0;
}