Codeforces977E——Cyclic Components

You are given an undirected graph consisting of n vertices and m edges. Your task is to find the number of connected components which are cycles.
Here are some definitions of graph theory.
An undirected graph consists of two sets: set of nodes (called vertices) and set of edges. Each edge connects a pair of vertices. All edges are bidirectional (i.e. if a vertex a is connected with a vertex b, a vertex b is also connected with a vertex a). An edge can’t connect vertex with itself, there is at most one edge between a pair of vertices.
Two vertices u and v belong to the same connected component if and only if there is at least one path along edges connecting u and v.
A connected component is a cycle if and only if its vertices can be reordered in such a way that:
the first vertex is connected with the second vertex by an edge,
the second vertex is connected with the third vertex by an edge,
…
the last vertex is connected with the first vertex by an edge,
all the described edges of a cycle are distinct.
A cycle doesn’t contain any other edges except described above. By definition any cycle contains three or more vertices.

There are 6 connected components, 2 of them are cycles: [7,10,16] and [5,11,9,15].
Input
The first line contains two integer numbers n and m (1≤n≤2⋅105, 0≤m≤2⋅105) — number of vertices and edges.
The following m lines contains edges: edge i is given as a pair of vertices vi, ui (1≤vi,ui≤n, ui≠vi). There is no multiple edges in the given graph, i.e. for each pair (vi,ui) there no other pairs (vi,ui) and (ui,vi) in the list of edges.
Output
Print one integer — the number of connected components which are also cycles.
Examples
Input
5 4
1 2
3 4
5 4
3 5
Output
1
Input
17 15
1 8
1 12
5 11
11 9
9 15
15 5
4 13
3 13
4 3
10 16
7 10
16 7
14 3
14 4
17 6
Output
2
Note
In the first example only component [3,4,5] is also a cycle.
The illustration above corresponds to the second example.

题意是给一个图，求单链环的个数，用dfs
建图的同时记录度数，然后在dfs的时候如果有度数不等于2的，标记一下！！ 不要直接退出，然后外边枚举搜索的第一个点，如果没有标记，则为一个环

代码：

#include <cstdio>
#include <algorithm>
#include <vector>
using namespace std;
const int N=200050;
int n,m;
vector<int> g[N];
int degree[N];
int a,b;
int cnt;
int ans;
bool vis[N];
int gidx[N];
int flag;
void dfs(int u){
    vis[u]=true;
    if(degree[u]!=2){
        flag=1;
    }
    int l=g[u].size();
    for(int i=0;i<l;i++){
        if(vis[g[u][i]]){
            continue;
        }
        dfs(g[u][i]);
    }
}
int main(void){
    //freopen("data.txt","r",stdin);
    scanf("%d%d",&n,&m);
    for(int i=0;i<m;i++){
        scanf("%d%d",&a,&b);
        g[a].push_back(b);
        g[b].push_back(a);
        degree[a]++;
        degree[b]++;
    }
    for(int i=1;i<=n;i++){
        flag=0;
        if(!vis[i]){
            dfs(i);
            if(!flag){
                ans++;
            }
        }
    }
    printf("%d\n",ans);
    return 0;
}