C# solution using a Trie

class TrieNode
{
    public TrieNode[] Child = new TrieNode[10];
    public bool WordEnd = false;
}

class Trie
{
    public TrieNode Root = new TrieNode();
    
    // Inserts s into the Trie and also checks if s is a prefix or has a prefix in the Trie.
    public bool Insert(string s)
    {
        bool hasPrefix = false;
        bool isPrefix = true;
        TrieNode currentNode = this.Root;
        
        foreach(char ch in s)
        {
            if (currentNode.WordEnd)
            {
                hasPrefix = true;
            }
            
            int index = ch - 'a';
            
            if (currentNode.Child[index] == null)
            {
                currentNode.Child[index] = new();
                isPrefix = false;
            }
            
            currentNode = currentNode.Child[index];
        }
        
        currentNode.WordEnd = true;
        
        return hasPrefix || isPrefix;
    }
}

class Result
{
    public static void noPrefix(List<string> words)
    {
        Trie trie = new();
        
        foreach (string word in words)
        {
            bool prefix = trie.Insert(word);
            
            if (prefix)
            {
                Console.WriteLine("BAD SET");
                Console.WriteLine(word);
                return;
            }
        }
        
        Console.WriteLine("GOOD SET");
    }
}

class TrieNode: def init(self): self.is_terminal = False self.children = [None] * 10 #alphabet ends at j

class Trie: def init(self): self.root = TrieNode()

def noPrefix(words): trie = Trie() alphabet = string.ascii_lowercase

for word in words:
    current_node = trie.root
    for char in word:
        i = alphabet.index(char)
        if current_node.children[i] is None:
            current_node.children[i] = TrieNode()

        current_node = current_node.children[i]

        if current_node.is_terminal:
            print('BAD SET')
            print(word)
            return

    current_node.is_terminal = True

    for child in current_node.children:
        if child is not None:
            print('BAD SET')
            print(word)
            return

print('GOOD SET')
return

One more c++ on Prefix Tree.

class OrderSet{ struct Tries{ unordered_map children; bool endOfWorld;

Tries(): endOfWorld(false){}; };

public:
Tries * root; OrderSet(): root (new Tries()){};

void insert(string str){ Tries * current = root; for(auto c: str){ if(current->children.find(c) == current->children.end()){ current->children[c]= new Tries(); } current = current->children[c]; } current->endOfWorld=true;
}

bool insertCheck(string str){ Tries * current = root; for(int i=0; i < str.size(); i++){ if(current->children.find(str[i]) == current->children.end()){ current->children[str[i]]= new Tries(); } else{ if(i+1 == str.size()) return false; } if(current->endOfWorld){ return false; } current = current->children[str[i]]; } current->endOfWorld=true; return true;
}

};

void noPrefix(vector words) {

OrderSet tree; tree.insert(words[0]); int i; bool bad_set=false; for(i=1; i < words.size(); i++){ if(!tree.insertCheck(words[i])){ bad_set = true; break; }
}

if(bad_set){ cout << "BAD SET" << endl; cout << words[i] << endl; } else { cout << "GOOD SET" << endl; } }

OMG the description is so confusing and wrong!!! Totally wrong about what they want to see. HackerRank has so much ambiguity and yet they are a bottleneck of code screening for many companies. This task is about building a Trie (Prefix tree) and looking for existing words. Nothing about word prefixes. Example 'as', 'sfdhk', 's' Will result in BAD SET "s", not "sfdhk".