POJ-2186 Popular Cows 强连通分量

　　题目链接：http://poj.org/problem?id=2186

　　求出度为0的强连通分量的点的个数。

  1 //STATUS:C++_AC_94MS_920KB

  2 #include <functional>

  3 #include <algorithm>

  4 #include <iostream>

  5 //#include <ext/rope>

  6 #include <fstream>

  7 #include <sstream>

  8 #include <iomanip>

  9 #include <numeric>

 10 #include <cstring>

 11 #include <cassert>

 12 #include <cstdio>

 13 #include <string>

 14 #include <vector>

 15 #include <bitset>

 16 #include <queue>

 17 #include <stack>

 18 #include <cmath>

 19 #include <ctime>

 20 #include <list>

 21 #include <set>

 22 #include <map>

 23 using namespace std;

 24 //define

 25 #define pii pair<int,int>

 26 #define mem(a,b) memset(a,b,sizeof(a))

 27 #define lson l,mid,rt<<1

 28 #define rson mid+1,r,rt<<1|1

 29 #define PI acos(-1.0)

 30 //typedef

 31 typedef __int64 LL;

 32 typedef unsigned __int64 ULL;

 33 //const

 34 const int N=10010;

 35 const int INF=0x3f3f3f3f;

 36 const int MOD=100000,STA=8000010;

 37 const LL LNF=1LL<<60;

 38 const double EPS=1e-8;

 39 const double OO=1e15;

 40 const int dx[4]={-1,0,1,0};

 41 const int dy[4]={0,1,0,-1};

 42 //Daily Use ...

 43 inline int sign(double x){return (x>EPS)-(x<-EPS);}

 44 template<class T> T gcd(T a,T b){return b?gcd(b,a%b):a;}

 45 template<class T> T lcm(T a,T b){return a/gcd(a,b)*b;}

 46 template<class T> inline T Min(T a,T b){return a<b?a:b;}

 47 template<class T> inline T Max(T a,T b){return a>b?a:b;}

 48 template<class T> inline T Min(T a,T b,T c){return min(min(a, b),c);}

 49 template<class T> inline T Max(T a,T b,T c){return max(max(a, b),c);}

 50 template<class T> inline T Min(T a,T b,T c,T d){return min(min(a, b),min(c,d));}

 51 template<class T> inline T Max(T a,T b,T c,T d){return max(max(a, b),max(c,d));}

 52 //End

 53 

 54 struct Edge{

 55     int u,v;

 56 }e[N*5];

 57 int first[N],next[N*5],pre[N],sccno[N],low[N],vis[N],p[N];

 58 int n,m,mt,dfs_clock,scnt;

 59 stack<int> s;

 60 

 61 int find(int x){return p[x]==x?x:p[x]=find(p[x]);}

 62 

 63 void adde(int a,int b)

 64 {

 65     e[mt].u=a;e[mt].v=b;

 66     next[mt]=first[a],first[a]=mt++;

 67 }

 68 

 69 void dfs(int u)

 70 {

 71     int i,j,v;

 72     pre[u]=low[u]=++dfs_clock;

 73     s.push(u);

 74     for(i=first[u];i!=-1;i=next[i]){

 75         v=e[i].v;

 76         if(!pre[v]){

 77             dfs(v);

 78             low[u]=Min(low[u],low[v]);

 79         }

 80         else if(!sccno[v]){

 81             low[u]=Min(low[u],low[v]);

 82         }

 83     }

 84     if(low[u]==pre[u]){

 85         int x=-1;

 86         scnt++;

 87         while(x!=u){

 88             x=s.top();s.pop();

 89             sccno[x]=scnt;

 90         }

 91     }

 92 }

 93 

 94 int main()

 95 {

 96  //   freopen("in.txt","r",stdin);

 97     int i,j,a,b,ans,x,y,ok;

 98     while(~scanf("%d%d",&n,&m))

 99     {

100         mem(first,-1);mt=0;

101         for(i=1;i<=n;i++)p[i]=i;

102         for(i=0;i<m;i++){

103             scanf("%d%d",&a,&b);

104             x=find(a);y=find(b);

105             if(x!=y)p[y]=p[x];

106             adde(a,b);

107         }

108         ok=0;

109         for(i=1;i<=n;i++){

110             if(p[i]==i)ok++;

111             if(ok>=2)break;

112         }

113         ans=0;

114         if(ok==1){

115             mem(pre,0);mem(sccno,0);

116             scnt=dfs_clock=0;

117             for(i=1;i<=n;i++){

118                 if(!pre[i])dfs(i);

119             }

120             for(i=1;i<=scnt;i++)vis[i]=1;

121             for(i=0;i<mt;i++){

122                 if(sccno[e[i].u]!=sccno[e[i].v]){

123                     vis[sccno[e[i].u]]=0;

124                 }

125             }

126             ok=0;

127             for(i=1;i<=scnt;i++){

128                 ok+=vis[i];

129                 if(ok>=2){ok=0;break;}

130             }

131             if(ok){

132                 for(i=1;i<=n;i++){

133                     if(vis[sccno[i]])ans++;

134                 }

135             }

136         }

137 

138         printf("%d\n",ans);

139     }

140     return 0;

141 }