有环图求环的个数和具体节点数_闭环个数怎么算

这个问题一直没仔细写过，cf上做到了就写一下，就是用栈存储+回溯，很简单。

#include<iostream>
#include<cstring>
#include<stack>
#include<vector>
#include<algorithm>
using namespace std;
const int N = 20;
vector<int> edge[N];
int s[N],top=0;//stl里的stack没办法遍历，所以用数组模拟
bool instack[N];
int cnt = 0;
 
void dfs(int u)
{
	s[top++] = u;
	instack[u] = true;
	for (int i = 0; i < edge[u].size(); i++)
	{
		int v = edge[u][i];
		if (!instack[v])
			dfs(v);
		else
		{
			printf("第%d个环: ", ++cnt);
			int t;
			for (t = top; s[t] != v; t--);
			for (int i = t; i < top; i++)
				cout << s[i] << " ";
			cout << endl;
		}
	}
	top--;		//回溯
	instack[u] = false;
}
 
int main()
{
	int n, m,u,v;
	cin >> n >> m;
	for (int i = 0; i < m; i++)
	{
		cin >> u >> v;
		edge[u].push_back(v);
	}
	dfs(1);
	return 0;
}