Ultraviolet A 11770 lighting away

Runid	User	Problem	Result	Memory	Time	Language	Length	Submit time
2482977	Zhyfzy	J	Accepted	0 KB	138 MS	C ++ 4.8.2	2322 B	2014-07-24 15:18:54

[Topic]

A directed graph triggers a chain reaction for each node operation, so that the node and its direct or indirect points are marked, ask how many nodes need to be operated to mark all nodes

[Question]

Point Reduction + DFS

First, we can think that operations must be performed on points with an inbound degree of 0. However, points with an inbound degree of 0 do not necessarily mark all nodes. For example, if a ring exists, we need to first scale down the vertex and use the Tarjan algorithm. For details, see the code. After the scale-down, we can directly count the number of vertices whose input degree is 0.

The Code uses the storage structure of the chain forward star

[Code]

#include<cstdio>#include<cstring>#include<cmath>#include<iostream>#include<stack>#define P 20000#define E 200000using namespace std;int i,j,k,n,T,ans,K,m,x,y,indexs,nn,mm;int ru[P],head[P],head2[P],dfn[P],low[P],instack[P],belong[P];bool b[E];stack <int> tar; struct node{int from;int to;int next;}map[E],map2[E];void addedge(int num,int x,int y){map[num].from=x;map[num].to=y;map[num].next=head[x];head[x]=num;}void addedge2(int num,int x,int y){//printf("NewEdge %d %d\n",x,y); ru[y]++;map2[num].from=x;map2[num].to=y;map2[num].next=head2[x];head2[x]=num;}void dfs(int p){for (int i=head2[p];i!=-1;i=map2[i].next){if (!b[map2[i].to]){b[map2[i].to]=true;dfs(map2[i].to);}}}void tarjan(int k){    int p;    tar.push(k);    instack[k]=1;    dfn[k]=low[k]=++indexs;    for(int j=head[k];j!=-1;j=map[j].next)    {        p=map[j].to;        if (instack[p]) {low[k]=min(low[k],dfn[p]);}        else if(dfn[p]==-1)        {            tarjan(p);            low[k]=min(low[k],low[p]);        }    }    if(low[k]==dfn[k])    {        nn++;        do        {            j=tar.top();            tar.pop();            instack[j]=0;            belong[j]=nn;        }while(j!=k);    }}void build_new_map(){    for(int i=1;i<=m;i++)    {        if(belong[map[i].from]==belong[map[i].to]) continue;        addedge2(++mm,belong[map[i].from],belong[map[i].to]);    }}void build_map(){scanf("%d%d",&n,&m);for (i=1;i<=m;i++){scanf("%d%d",&x,&y);addedge(i,x,y);}memset(dfn,-1,sizeof(dfn));memset(low,-1,sizeof(low));memset(instack,0,sizeof(instack));indexs=0;nn=0;for (i=1;i<=n;i++) belong[i]=i;for (i=1;i<=n;i++){if (dfn[i]==-1)tarjan(i);}build_new_map();}int main(){scanf("%d",&T);while (++K<=T){memset(map,-1,sizeof(map));memset(head,-1,sizeof(head));memset(map2,-1,sizeof(map2));memset(head2,-1,sizeof(head2));memset(b,0,sizeof(b));memset(ru,0,sizeof(ru));ans=0;mm=0;nn=0;build_map();for (i=1;i<=nn;i++){if (!ru[i]){ans++;b[i]=true;dfs(i);}}for (i=1;i<=nn;i++){if (!b[i]){ans++; b[i]=true;dfs(i);}}printf("Case %d: %d\n",K,ans);}}