T262837 简单的操作（merge）

https://www.luogu.com.cn/record/85457101

Luogu/T262837/T262837.cpp

@@ -0,0 +1,119 @@
+#include <iostream>
+#include <queue>
+#include <stack>
+#include <unordered_set>
+#include <utility>
+#include <vector>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 1005;
+
+int n, m, color[N], ans;
+int cnt, id[N];
+bool vis[N];
+std::vector<int> g[N];
+std::unordered_set<int> p[N];
+
+bool dfs1(int u, int c) {
+    color[u] = c;
+
+    for (int v : g[u]) {
+        if (color[v]) {
+            if (color[v] == c) return false;
+        } else if (!dfs1(v, 3 - c)) {
+            return false;
+        }
+    }
+
+    return true;
+}
+
+bool check() {
+    for (int i = 1; i <= n; i++) {
+        if (!color[i]) {
+            if (!dfs1(i, 1)) return false;
+        }
+    }
+
+    return true;
+}
+
+void dfs2(int u, int x) {
+    id[u] = x;
+    p[x].insert(u);
+
+    for (int v : g[u]) {
+        if (!id[v]) dfs2(v, x);
+    }
+}
+
+int bfs(int x) {
+    std::fill_n(vis, N, false);
+
+    int res = 0;
+
+    std::queue<std::pair<int, int>> q;
+
+    q.emplace(x, 0);
+    vis[x] = true;
+
+    while (!q.empty()) {
+        auto e = q.front();
+        q.pop();
+
+        int u = e.first,
+            w = e.second;
+
+        res = std::max(res, w);
+
+        for (int v : g[u]) {
+            if (vis[v]) continue;
+            vis[v] = true;
+
+            q.emplace(v, w + 1);
+        }
+    }
+
+    return res;
+}
+
+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].emplace_back(v);
+        g[v].emplace_back(u);
+    }
+
+    if (!check()) {
+        cout << -1 << endl;
+
+        exit(0);
+    }
+
+    for (int i = 1; i <= n; i++) {
+        if (!id[i]) dfs2(i, ++cnt);
+    }
+
+    for (int i = 1; i <= cnt; i++) {
+        int res = 0;
+
+        for (const int &x : p[i]) {
+            res = std::max(res, bfs(x));
+        }
+
+        ans += res;
+    }
+
+    cout << ans << endl;
+
+    return 0;
+}