P5022 [NOIP2018 提高组] 旅行

https://www.luogu.com.cn/record/77062666

Luogu/P5022/P5022.cpp

@@ -0,0 +1,103 @@
+#include <iostream>
+#include <algorithm>
+#include <queue>
+#include <unordered_map>
+#include <vector>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 5005;
+
+int n, m, s, t, in[N];
+bool vis[N];
+std::vector<int> c, g[N];
+std::unordered_map<int, int> map[N];
+
+void cycle() {
+    std::queue<int> q;
+
+    for (int i = 1; i <= n; i++) {
+        if (in[i] == 1) q.push(i);
+    }
+
+    while (!q.empty()) {
+        int u = q.front();
+        q.pop();
+
+        for (int v : g[u]) {
+            if (in[v] > 1) {
+                if (--in[v] == 1) q.push(v);
+            }
+        }
+    }
+}
+
+void dfs(int u, int fa, std::vector<int> &res) {
+    if (vis[u]) return;
+    vis[u] = true;
+
+    res.push_back(u);
+
+    for (int v : g[u]) {
+        if (v == fa || u == s && v == t || v == s && u == t) continue;
+
+        dfs(v, u, res);
+    }
+
+    vis[u] = false;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n >> m;
+
+    for (int i = 1, u, v; i <= m; i++) {
+        cin >> u >> v;
+
+        g[u].push_back(v);
+        g[v].push_back(u);
+        in[u]++, in[v]++;
+
+        map[u][v] = map[v][u] = true;
+    }
+
+    cycle();
+
+    for (int i = 1; i <= n; i++) {
+        std::sort(g[i].begin(), g[i].end());
+        if (in[i] >= 2) c.push_back(i);
+    }
+
+    std::vector<int> ans;
+
+    if (n == m) {
+        ans = std::vector<int>(n, n);
+
+        for (int i = 0; i < c.size(); i++) {
+            for (int j = i + 1; j < c.size(); j++) {
+                s = c[i], t = c[j];
+
+                if (!map[s][t]) continue;
+
+                std::vector<int> res;
+
+                dfs(1, 0, res);
+
+                ans = std::min(ans, res);
+            }
+        }
+    } else {
+        dfs(1, 0, ans);
+    }
+
+    for (int &ans : ans) {
+        cout << ans << ' ';
+    }
+    cout << endl;
+
+    return 0;
+}