【题解】CF977E：Cyclic Components

原题传送门
求环的个数
直接dfs处理出每个连通块
然后如果一个连通块的所有点的度数都为2，那么是一个环
Code：

#include <bits/stdc++.h>
#define maxn 200010
using namespace std;
struct Edge{
	int to, next;
}edge[maxn << 1];
int num, head[maxn], n, m, d[maxn], vis[maxn], ans, cnt;
vector <int> loop[maxn];

inline int read(){
	int s = 0, w = 1;
	char c = getchar();
	for (; !isdigit(c); c = getchar()) if (c == '-') w = -1;
	for (; isdigit(c); c = getchar()) s = (s << 1) + (s << 3) + (c ^ 48);
	return s * w;
}

void addedge(int x, int y){ edge[++num] = (Edge){y, head[x]}; head[x] = num; }

void dfs(int u){
	loop[cnt].push_back(u); vis[u] = 1;
	for (int i = head[u]; i; i = edge[i].next){
		int v = edge[i].to;
		if (!vis[v]) dfs(v);
	}
}

int main(){
	n = read(), m = read();
	for (int i = 1; i <= m; ++i){
		int x = read(), y = read();
		addedge(x, y), addedge(y, x);
		++d[x], ++d[y];
	}
	for (int i = 1; i <= n; ++i)
		if (!vis[i]) ++cnt, dfs(i);
	for (int i = 1; i <= cnt; ++i){
		int flag = 1;
		for (int j = 0; j < loop[i].size(); ++j)
			if (d[loop[i][j]] != 2){ flag = 0; break; }
		ans += flag;
	}
	printf("%d\n", ans);
	return 0;
}