【POJ】3740 Easy Finding 精确覆盖入门题

传送门:【POJ】3740 Easy Finding


题目分析:Dancing Links 模板题。


代码如下:


#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std ;

#define REP( i , a , b ) for ( int i = a ; i < b ; ++ i )
#define REV( i , a , b ) for ( int i = a - 1 ; i >= b ; -- i )
#define FOR( i , a , b ) for ( int i = a ; i <= b ; ++ i )
#define FOV( i , a , b ) for ( int i = a ; i >= b ; -- i )
#define REC( i , A , o ) for ( int i = A[o] ; i != o ; i = A[i] )
#define CLR( a , x ) memset ( a , x , sizeof a )

const int MAXN = 305 ;
const int MAXNODE = 5000 ;

struct DLX {
	int L[MAXNODE] , R[MAXNODE] , U[MAXNODE] , D[MAXNODE] ;
	int row[MAXNODE] , col[MAXNODE] ;
	int S[MAXN] , H[MAXN] ;
	int deep , ans[MAXN] ;
	int size ;
	int n , m ;
	
	void remove ( int c ) {
		L[R[c]] = L[c] ;
		R[L[c]] = R[c] ;
		REC ( i , D , c )
			REC ( j , R , i ) {
				U[D[j]] = U[j] ;
				D[U[j]] = D[j] ;
				-- S[col[j]] ;
			}
	}
	
	void resume ( int c ) {
		REC ( i , U , c )
			REC ( j , L , i ) {
				++ S[col[j]] ;
				D[U[j]] = j ;
				U[D[j]] = j ;
			}
		R[L[c]] = c ;
		L[R[c]] = c ;
	}
	
	int dance ( int d ) {
		if ( R[0] == 0 ) {
			deep = d ;
			return 1 ;
		}
		int c = R[0] ;
		REC ( i , R , 0 )
			if ( S[c] > S[i] )
				c = i ;
		remove ( c ) ;
		REC ( i , D , c ) {
			ans[d] = row[i] ;
			REC ( j , R , i )
				remove ( col[j] ) ;
			if ( dance ( d + 1 ) )
				return 1 ;
			REC ( j , L , i )
				resume ( col[j] ) ;
		}
		resume ( c ) ;
		return 0 ;
	}
	
	void link ( int r , int c ) {
		++ size ;
		row[size] = r ;
		col[size] = c ;
		++ S[c] ;
		U[size] = U[c] ;
		D[size] = c ;
		D[U[c]] = size ;
		U[c] = size ;
		if ( ~H[r] ) {
			R[size] = H[r] ;
			L[size] = L[H[r]] ;
			L[R[size]] = size ;
			R[L[size]] = size ;
		}
		else
			H[r] = L[size] = R[size] = size ;
	}
	
	void init () {
		CLR ( H , -1 ) ;
		FOR ( i , 0 , n ) {
			R[i] = i + 1 ;
			L[i] = i - 1 ;
			U[i] = i ;
			D[i] = i ;
		}
		R[n] = 0 ;
		L[0] = n ;
		size = n ;
	}
	
	int read () {
		char c = ' ' ;
		while ( c < '0' || c > '9' )
			c = getchar () ;
		int x = c - '0' ;
		getchar () ;
		return x ;
	}
	
	void input () {
		FOR ( i , 1 , m )
			FOR ( j , 1 , n )
				if ( read () )
					link ( i , j ) ;
	}
	
	void solve () {
		init () ;
		input () ;
		if ( dance ( 0 ) )
			printf ( "Yes, I found it\n" ) ;
		else
			printf ( "It is impossible\n" ) ;
	}
} dlx ;

int main () {
	while ( ~scanf ( "%d%d" , &dlx.m , &dlx.n ) )
		dlx.solve () ;
	return 0 ;
}


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