#include <vector>
#include <cstdint>

struct edge_list
{
    typedef std::size_t size_type;

    // Node
    struct node
    {
        node( unsigned value, unsigned next ) :
            value( value ),
            next( next )
        {
        }
        size_type value;
        size_type next;
    };

    // Resize the list
    void resize( size_type n, size_type data = 0 )
    {
        edges_.resize( n, 0 );
        data_.reserve( data * 2 );
    }

    // Add a new edge
    void add_edge( size_type a, size_type b )
    {
        add_edge_internal( a, b );
        add_edge_internal( b, a );
    }

    // Each
    template< typename Functor >
    void each( size_type i, Functor fn ) const
    {
        size_type n = edges_[ i ];
        while ( n != 0 )
        {
            const node& ref = data_[ n - 1 ];
            n = ref.next;
            fn( ref.value - 1 );
        }
    }

private:
    // Add edge internal
    void add_edge_internal( size_type a, size_type b )
    {
        size_type old_top = edges_[ a ];
        data_.emplace_back( b + 1, old_top );
        edges_[ a ] = data_.size();
    }

private:
    std::vector< node >      data_;
    std::vector< size_type > edges_;
};