Hey,  

Check out how I implemented problem 55 - Companion plants 2: 

public Boolean companionPlants(List<String> plants) { 

    //code here 

    if(plants.size()==1){ 

        return false; 

    } 

    Map<String, List<String>> companionMap = new Map<String, List<String>>(); 

    companionMap.put('lettuce', new List<String>{'cucumbers', 'onions'});  

    companionMap.put('onions', new List<String>{'lettuce', 'carrots', 'tomatoes'}); 

    companionMap.put('cucumbers', new List<String>{'lettuce'}); 

    companionMap.put('carrots', new List<String>{'onions'}); 

    companionMap.put('tomatoes', new List<String>{'onions'}); 

    for(Integer i = 0; i < plants.size()-1; i++){ 

        List<String> currentCompanions = companionMap.get(plants[i]); 

        if(!currentCompanions.contains(plants[i+1])){ 

            return false; 

        } 

    } 

    return true; 

} 

Now I am in top 4 percent of all ApexSandbox users.