poj2942解题报告

这道题其实就是求割点数量

用点双联通

#include <iostream>
#include <cstring>
#include <cstdio>
#include <stack>
using namespace std ;

const int MAXN = 1001 ; 
const int MAXM = 2*MAXN * MAXN ; 
const int INF = 99999999 ; 

struct type 
{
	int  end , next ;
} ; 

type  edge[MAXM] ; 
int   head[MAXN] , Count , map[MAXN][MAXN] , vis[MAXN] , visit[MAXN] , low[MAXN] , dfn[MAXN] ;
int   color[MAXN] , belone[MAXN] , val[MAXN] , ans , cnt , Time , flag ; 
stack < int > S ; 

void  addedge( int start , int end )
{
	edge[Count].end = end ; 
	edge[Count].next = head[start] ; 
	head[start] = Count ++ ;
}

void  dfs( int root , int col )
{
	int  i , end ; 
	color[root] = col ; 
	for( i = head[root] ; i != -1 ; i = edge[i].next )
	{
		end = edge[i].end ;
		if( belone[end] != cnt ) continue ;
		if( color[end] == 1 - col ) continue ;
		if( color[end] == -1 ) dfs( end , 1 - col ) ;
		else if( color[end] == col ) flag = 1 ;
		if( flag ) return ;
	}
}

void  tarjan_dfs( int cur , int father  )
{
	S.push( cur ) ;  
	dfn[cur] = low[cur] = Time ++ ; 
	visit[cur] = 1 ; 
	for( int i = head[cur] ; i != -1 ; i = edge[i].next )
	{
		int  end = edge[i].end ; 
		if( end == father ) continue ; 
		if( !visit[end] )
		{
			tarjan_dfs( end , cur ) ; 
			low[cur] = min( low[cur] , low[end] ) ; 
			if( low[end] >= dfn[cur] )
			{
				int  ss ; 
				cnt ++ ;
				ans = 0 ; 
				do
				{
					ss = S.top() ;
					S.pop() ;
					val[++ans] = ss ; 
					belone[ss] = cnt ; 
				}while( ss != end ) ; 
				belone[cur] = cnt ; 
				val[++ans] = cur ;
				flag = 0 ; 
				memset( color , -1 , sizeof( color ) ) ;
				dfs( cur , 0 ) ; 
				if( flag ) for( int j = 1 ; j <= ans ; j ++ )
					vis[val[j]] = 1 ;
			}
		}
		else low[cur] = min( low[cur] , dfn[end] ) ; 
	}
}

int main()
{
	int  n , m , i , j , temp , sum ; 
	while( scanf( "%d%d" , & n , & m ) != EOF && n && m )
	{
		memset( head , -1 , sizeof( head ) ) ; 
		memset( map , 0 , sizeof( map ) ) ;
		Count = 0 ; 
		while( m -- ) 
		{
			int  start , end ; 
			scanf( "%d%d" , & start , & end ) ; 
			map[start][end] = map[end][start] = 1 ; 
		}
		for( i = 1 ; i <= n ; i ++ )
			for( j = i + 1 ; j <= n ; j ++ )
				if( ! map[i][j] ){
					addedge( i , j ) ; 
					addedge( j , i ) ; 
				}
		cnt = 0 ; Time = 0 ;  
		memset( vis , 0 , sizeof( vis ) ) ; 
		memset( visit , 0 , sizeof( visit ) ) ;
		memset( belone , -1 , sizeof( belone ) ) ;
		for( int i = 1 ; i <= n ; i ++ )
			if( ! visit[i] ) tarjan_dfs( i , -1 ) ;
		sum = 0 ; 
		for( int i = 1 ; i <= n ; i ++ )
			if( vis[i] ) sum ++ ; 
		printf( "%d\n" , n - sum ) ; 
	}
	return 0 ; 
}