codeforces #80div2 C【無向圖判單環】

腦殘，竟然水不過！

題意很簡單，就是判斷無向圖是否有單環。

方法：dfs給每一點染色，最後看是否還有點沒有染色，如果有的話，就NO，否則就FHTAGN!。注意，當n！=m時就輸出NO，因為有結論：如果存在單環，則點數=邊數。

#include <vector>#include <list>#include <map>#include <set>#include <queue>#include <string.h>#include <deque>#include <stack>#include <bitset>#include <algorithm>#include <functional>#include <numeric>#include <utility>#include <sstream>#include <iostream>#include <iomanip>#include <cstdio>#include <cmath>#include <cstdlib>#include <limits.h>using namespace std;int lowbit(int t){return t&(-t);}int countbit(int t){return (t==0)?0:(1+countbit(t&(t-1)));}int gcd(int a,int b){return (b==0)?a:gcd(b,a%b);}int max(int a,int b){return a>b?a:b;}int min(int a,int b){return a>b?b:a;}#define LL long long#define pi acos(-1)#define N  1000010#define INF INT_MAX#define eps 1e-8#define FRE freopen("a.txt","r",stdin)vector<int> v[110];int vis[110];void dfs(int cur){    int i;    vis[cur]=1;    for(i=0;i<v[cur].size();i++)    {        int tmp=v[cur][i];        if(vis[tmp]==0)        {            dfs(tmp);        }    }}int main(){    int n,m;    while(scanf("%d%d",&n,&m)!=EOF)    {        int i,j;        for(i=0;i<=n;i++)v[i].clear();        for(i=0;i<m;i++)        {            int a,b;            scanf("%d%d",&a,&b);            v[a].push_back(b);            v[b].push_back(a);        }        if(n!=m)//這裡已經篩選出沒環、多環(只有一條路徑)        {            printf("NO\n");        }        else//對多條路徑判斷        {            for(i=0;i<=n;i++)vis[i]=0;            dfs(1);            for(i=1;i<=n;i++)            if(!vis[i])break;            if(i>n)printf("FHTAGN!\n");            else            printf("NO\n");        }    }    return 0;}