D - Neighbors

https://atcoder.jp/contests/abc231/submissions/27839591

AtCoder/ABC231/D/D.cpp

@@ -0,0 +1,46 @@
+#include <cstdio>
+#include <cstdlib>
+#include <vector>
+
+int n, m, a, b;
+bool vis1[100005], vis2[100005];
+std::vector<int> g[100005];
+
+void dfs(int u, int f) {
+    vis1[u] = 1;
+    if (vis2[u]) {
+        printf("No\n");
+        exit(0);
+    }
+    vis2[u] = true;
+    for (int v : g[u]) {
+        if (v == f) continue;
+        dfs(v, u);
+    }
+    vis2[u] = false;
+}
+
+int main() {
+    scanf("%d%d", &n, &m);
+    if (m >= n) {
+        printf("No\n");
+        exit(0);
+    }
+    for (int i = 1; i <= m; i++) {
+        scanf("%d%d", &a, &b);
+        g[a].push_back(b);
+        g[b].push_back(a);
+    }
+    for (int i = 1; i <= n; i++) {
+        if (!vis1[i])
+            dfs(i, i);
+    }
+    for (int i = 1; i <= n; i++) {
+        if (g[i].size() > 2) {
+            printf("No\n");
+            exit(0);
+        }
+    }
+    printf("Yes\n");
+    return 0;
+}