村子（最小化）

解题：贪心

很明显越靠近越好。随便从一个点出发，按照翻的排列方式来选择和父亲链接还是和兄弟链接。记得每次加2哦~~~~

具体代码：

#include<bits/stdc++.h>
using namespace std;
#define int long long
const int N = 1e5 + 500;
int n, par[N], swp[N], p[N];
vector<int>g[N], rev;
void dfs(int a, int pp) {
    rev.push_back(a);
    par[a] = pp;
    for (auto& x : g[a]) {
        if (x == pp)continue;
        dfs(x, a);
    }
}
signed main() {
    ios_base::sync_with_stdio(0); cin.tie(0);
    int n; cin >> n;
    for (int i = 0; i < n - 1; i++) {
        int u, v; cin >> u >> v;
        g[u].push_back(v); g[v].push_back(u);
    }
    for (int i = 1; i <= n; i++) p[i] = i;
    dfs(1, 0);
    reverse(rev.begin(), rev.end());
    int ans = 0;
    for (int i = 0; i < n; i++) {
        int u = rev[i];
        if (!swp[u]) {
            int up = par[u];
            if (par[u] == 0) up = g[u][0];
            swp[u] = swp[up] = 1;
            swap(p[u], p[up]);
            ans += 2;
        }
    }
    cout << ans << '\n';
    for (int i = 1; i <= n; i++) cout << p[i] << ' ';
    cout << '\n';
}