Codeforces 104C Cthulhu dfs暴力 || 点双连通缩点

题目链接：点击打开链接

题意：

给定n个点m条边的无向图

问图中是否存在 有且仅有一个简单环和一些树，且这些树的root都在这个简单环上。

瞎写了个点双。

。

==

#include <stdio.h>
#include <iostream>
#include <algorithm>
#include <string.h>
#include <queue>
#include <vector>
#include <set>
using namespace std;
#define N 105
#define M 100005
#define inf 10000000
struct Edge{
int from,to,next;
}edge[2*M];
int head
,edgenum;
int Low
,DFN
,Stack
;//Belong数组的值是1~block
int Index,top;
int Belong
,block;//新图的连通块标号（1~block）
bool Instack
;
int bridge; //割桥数量
vector<int> bcc
;
void addedge(int u,int v){
Edge E={u,v,head[u]}; edge[edgenum]=E; head[u] = edgenum++;
Edge E2={v,u,head[v]};edge[edgenum]=E2;head[v] = edgenum++;
}
void Tarjan(int u,int pre){
int v;
Low[u] = DFN[u] = ++Index;
Stack[top++] = u;
Instack[u] = true;
for(int i = head[u]; ~i ;i = edge[i].next){
v = edge[i].to;
// 假设重边有效的话以下这句改成: if(v == pre && pre_num == 0){pre_num++;continue;} pre_num在for上面定义 int pre_num=0;
if( v == pre )continue;
if( !DFN[v] ){
Tarjan(v,u);
if(Low[u] > Low[v])Low[u] = Low[v];
if(Low[v] > Low[u])
bridge++;
}
else if(Instack[v] && Low[u] > DFN[v])Low[u] = DFN[v];
}
if(Low[u] == DFN[u]){
block++;
bcc[block].clear();
do{
v = Stack[--top];
Instack[v] = false;
Belong[v] = block;
bcc[block].push_back(v);
}while( v != u );
}
}
void work(int l, int r){
memset(DFN,0,sizeof(DFN));
memset(Instack,false,sizeof(Instack));
Index = top = block = bridge = 0;
for(int i = l; i <= r; i++)if(!DFN[i])Tarjan(i,i);
}
vector<int>G
;
void suodian(){
for(int i = 1; i <= block; i++)G[i].clear();
for(int i = 0; i < edgenum; i+=2){
int u = Belong[edge[i].from], v = Belong[edge[i].to];
if(u==v)continue;
G[u].push_back(v), G[v].push_back(u);
}
}
void init(){edgenum = 0; memset(head,-1,sizeof(head));}

int n, m, vis
, siz
, f
;
int find(int x){return x==f[x]?x:f[x]=find(f[x]);}
void Union(int x, int y){
int fx = find(x), fy = find(y);
if(fx==fy)return;
if(fx>fy)swap(fx,fy); f[fx] = fy;
}
bool dfs(int u){
vis[u] = 1;
bool ok = true;
for(int i = 0; i < G[u].size(); i++)
{
int v = G[u][i]; if(vis[v])continue;
if(bcc[v].size()>1)ok = false;
if(dfs(v)==false)ok = false;
}
return ok;
}
set<int>myset;
bool solve(){
int i, u, v;
init();
for(i = 1; i <= n; i++)f[i] = i;
while(m--){
scanf("%d %d",&u,&v);
addedge(u, v);
Union(u,v);
}
myset.clear();
for(i = 1; i<= n; i++)myset.insert(find(i));
if(myset.size()>1)return false;
work(1, n);
suodian();
memset(vis, 0, sizeof vis);
memset(siz, 0, sizeof siz);
for(i = 0; i < edgenum; i+=2)
siz[u]+= (Belong[edge[i].from]==Belong[edge[i].to]);

for(i = 1; i <= block; i++) if(!vis[i] && bcc[i].size()>1 && bcc[i].size()==siz[i] && G[i].size()>=3)
if(dfs(i))return true;

return false;
}
int main(){
while(~scanf("%d %d", &n, &m))
solve() ? puts("FHTAGN!") : puts("NO");
return 0;
}
/*
6 6
1 2
2 3
3 1
4 5
5 6
6 4

*/