图论判环模板

#include <iostream>
#include <cstring>
#include <string>
#include <cmath>
#include <algorithm>
#include <cstdio>
#include <sstream>
#include <vector>

using namespace std;
const int MAX = 1e5 + 5;
int n, m, flag;
vector<int> v[MAX];
int vis[MAX];

inline void file()
{
freopen("D:\\go.txt", "r", stdin);
freopen("D:\\leave.txt", "w", stdout);
}

void dfs(int id, int fa)
{
if (vis[id])
{
flag = 1;
return ;
}
int lenth = v[id].size();
vis[id] = 1;
for (int i = 0; i < lenth; ++i)
{
if (v[id][i] == fa)
continue;
dfs(v[id][i], id);
}
}

int main()
{
//file();
scanf("%d%d", &n, &m);
int f, t;
while (m--)
{
scanf("%d%d", &f, &t);
v[f].push_back(t);
v[t].push_back(f);
}
int res = 0;
for (int i = 1; i <= n; ++i)
{
flag = 0;
if (vis[i] == 0)
{
dfs(i, -1);
if (!flag)
res++;
}

}
printf("%d\n", res);
return 0;
}