UVa 1326 - Jurassic Remains(枚举子集+中途相遇法)

训练指南p.59

#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <map>

using namespace std;

const int MAXN = 30;

int N;
int A[MAXN];
char str[1010];
map<int , int> table;

int bitcount( int x )
{
    int res = 0;
    while ( x )
    {
        if ( x & 1 ) ++res;
        x >>= 1;
    }
    return res;
}

int main()
{
    while ( scanf( "%d", &N ) == 1 )
    {
        for ( int i = 0; i < N; ++i )
        {
            scanf( "%s", str );
            A[i] = 0;
            for ( int j = 0; str[j]; ++j )
                A[i] |= ( 1 << ( str[j] - 'A' ) );
        }

        table.clear();

        int n1 = N / 2, n2 = N - n1;
        for ( int i = 0; i < ( 1 << n1 ); ++i )
        {
            int tmp = 0;
            for ( int j = 0; j < n1; ++j )
            if ( i & ( 1 << j ) )
                tmp ^= A[j];
            if ( !table.count(tmp) || bitcount(i) > bitcount( table[tmp] ) ) table[tmp] = i;
        }

        int ans = 0;
        for ( int i = 0; i < ( 1 << n2 ); ++i )
        {
            int tmp = 0;
            for ( int j = 0; j < n2; ++j )
            if ( i & ( 1 << j ) ) tmp ^= A[ n1 + j ];
            if ( table.count(tmp) && bitcount(ans) < bitcount(i) + bitcount(table[tmp]) )
                ans = ( i << n1 ) | table[tmp];
        }

        printf("%d\n", bitcount(ans) );
        bool first = false;
        for ( int i = 0; i < N; ++i )
        if ( ans & ( 1 << i ) )
        {
            if ( first ) putchar(' ');
            printf("%d", i + 1);
            first = true;
        }
        puts("");
    }
    return 0;
}