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;}