Reverses a mapping of map: T -> vector<S> to map: S -> vector<T>

map reverse

// Reverses a mapping of map: T -> vector<S> to map: S -> vector<T>
template<typename T, typename S>
void reverseMapping( map< T, vector<S> >& oMapping, map< S, vector<T> >& revMapping )
{
  for( typename map< T, vector<S> >::iterator iter = oMapping.begin() ; iter != oMapping.end() ; ++iter )
  {
    for( int i = 0 ; i < iter->second.size() ; i++ )
    {
      pair< typename map< S, vector<T> >::iterator, bool >   res = 
        revMapping.insert( make_pair( iter->second[i], vector<T>() ) ) ;
      
      res.first->second.push_back( iter->first ) ;
    }
  }
}