强连通 HDU 3861

  1 #include<stdio.h>
  2 #include<algorithm>
  3 #include<string.h>
  4 #include<stack>
  5 
  6 using namespace std;
  7 
  8 #define MAXN 5010
  9 #define MAXN1 100010
 10 int head[MAXN],dfn[MAXN],low[MAXN],cou[MAXN],fa[MAXN];
 11 int cnt,k,num;
 12 bool vis[MAXN],mark[MAXN];
 13 
 14 struct edg
 15 {
 16     int next,to,fr;
 17 }x[MAXN1];
 18 void add(int u,int v)
 19 {
 20     x[cnt].next=head[u];
 21     x[cnt].fr=u;
 22     x[cnt].to=v;
 23     head[u]=cnt++;
 24 }
 25 stack<int>s;
 26 
 27 void dfs(int u)
 28 {
 29     low[u]=dfn[u]=k++;
 30     vis[u]=1;
 31     s.push(u);
 32     int i;
 33     for(i=head[u];i!=-1;i=x[i].next)
 34     {
 35         int v=x[i].to;
 36         if(!dfn[v])
 37         {
 38             dfs(v);
 39             low[u]=min(low[u],low[v]);
 40         }
 41         else if(vis[v])
 42             low[u]=min(low[u],dfn[v]);
 43     }
 44     if(dfn[u]==low[u])
 45     {
 46         num++;
 47         while(!s.empty())
 48         {
 49             int now=s.top();
 50             s.pop();
 51             vis[now]=0;
 52             fa[now]=num;
 53             if(now==u)break;
 54         }
 55     }
 56 }
 57 int pa[MAXN];
 58 
 59 bool dfs1(int u)
 60 {
 61     int i;
 62     for(i=head[u];i!=-1;i=x[i].next)
 63     {
 64         if(mark[x[i].to])
 65             continue;
 66         mark[x[i].to]=1;
 67         if(pa[x[i].to]==-1||dfs1(pa[x[i].to]))
 68         {
 69             pa[x[i].to]=u;
 70             return 1;
 71         }
 72     }
 73     return 0; //很重要
 74 }
 75 int main()
 76 {
 77     int t;
 78     scanf("%d",&t);
 79 
 80     while(t--)
 81     {
 82         int n,m,i;
 83         scanf("%d%d",&n,&m);
 84         cnt=0;
 85         memset(head,-1,sizeof(head));
 86         memset(dfn,0,sizeof(dfn));
 87         memset(low,0,sizeof(low));
 88 
 89         for(i=1;i<=m;i++)
 90         {
 91             int a,b;
 92             scanf("%d%d",&a,&b);
 93             add(a,b);
 94         }
 95         k=1;
 96         num=0;
 97         for(i=1;i<=n;i++) //强联通
 98             if(!dfn[i])
 99                 dfs(i);
100         memset(head,-1,sizeof(head));
101         memset(pa,-1,sizeof(pa));
102         int en=cnt;
103         cnt=0;
104         for(i=0;i<en;i++)
105         {
106             int u,v;
107             u=fa[x[i].fr];
108             v=fa[x[i].to];
109             if(u!=v)
110             {
111                 add(u,v);
112             }
113         }
114         int ans=0;
115 
116         for(i=1;i<=num;i++) //二分匹配
117         {
118             memset(mark,0,sizeof(mark));
119             if(dfs1(i))
120                 ans++;
121         }
122         printf("%d\n",num-ans);
123     }
124 
125     return 0;
126 }