The O(n) Club: Trie (Prefix Tree): Where Your Prefix Issues Go To Die

The O(n) Club: Trie (Prefix Tree): Where Your Prefix Issues Go To Die

⚡ TL;DR

Tries, aka prefix trees, are the go-to when you want to answer annoying questions like “Does any word start with ‘auto’?” faster than your coffee gets cold. They keep your strings as tidy as a Marie Kondo Netflix special. If you’re bored, here’s the absolute basics in Java:

// Java TrieNode: minimal and unimpressed
class TrieNode {
    TrieNode[] children = new TrieNode[26];
    boolean isWord;
}

🧠 How Devs Usually Mess This Up

Some devs think tries are just binary search trees after a wild night out (spoiler: they’re not). Others imagine loading every word means magic space-saving. Reality check: if your data is more “unique snowflake” than “copy-paste,” tries will gobble memory like a Chrome tab hoarding RAM. And don’t forget the classic fail—assuming every node finishing a prefix is a real word. If you miss marking your word ends, get ready to play “Is ‘app’ a word or just the opening act for ‘apple’?” forever.

🔍 What’s Actually Going On

Picture a call center, but each agent only answers to one letter. You follow the path for “a-p-p-l-e”—if there’s no agent on the next line, your query dies. If you make it to a node with a “yes, this is a word” stamp, you win. Common prefixes share space (all “app” words use the same path at the start), so autocomplete and spell-check work like a dream—as long as your words actually use the same beginnings.

🛠️ PseudoCode

• Insert a Word:
  • Start at the root node.
  • For each character in your word:
    • If the child exists, walk there like you own the place.
    • If not, make a new node and stroll in.
  • Mark the last node as isWord = true (end-of-word badge earned).

  // Java-style
  void insert(String word) {
      TrieNode node = root;
      for (char c : word.toCharArray()) {
          if (node.children[c - 'a'] == null)
              node.children[c - 'a'] = new TrieNode();
          node = node.children[c - 'a'];
      }
      node.isWord = true;
  }

• Search a Word:
  • Start at the root, check characters one by one.
  • If any path is missing, answer is “nope”.
  • If you make it, return isWord—that’s your finish line.
• Check a Prefix:
  • Walk as above, but only care if you can traverse the entire prefix. Don’t care about isWord here.

💻 The Code

// Java Trie in action
class Trie {
    private static class TrieNode {
        TrieNode[] children = new TrieNode[26];
        boolean isWord;
    }
    private final TrieNode root = new TrieNode();
    public void insert(String word) {
        TrieNode node = root;
        for (char c : word.toCharArray()) {
            int idx = c - 'a';
            if (node.children[idx] == null)
                node.children[idx] = new TrieNode();
            node = node.children[idx];
        }
        node.isWord = true;
    }
    public boolean search(String word) {
        TrieNode node = root;
        for (char c : word.toCharArray()) {
            int idx = c - 'a';
            if (node.children[idx] == null)
                return false;
            node = node.children[idx];
        }
        return node.isWord;
    }
    public boolean startsWith(String prefix) {
        TrieNode node = root;
        for (char c : prefix.toCharArray()) {
            int idx = c - 'a';
            if (node.children[idx] == null)
                return false;
            node = node.children[idx];
        }
        return true;
    }
}

⚠️ Pitfalls, Traps & Runtime Smacks

• Case (in)sensitivity? This code is for lowercase ‘a’-‘z’ only. Give it an uppercase character, and watch it implode.
• Mega alphabets? If your trie only holds “cat” and “dog,” you’re wasting space for 24 letters at every node. Sorry, English isn’t always efficient.
• Null pointer drama: Forget to check for null? Get ready for runtime rants.
• Not a hashmap, not O(1): All main ops are O(length of word/prefix). Your speedy hash table is still faster for random lookup.

🧠 Interviewer Brain-Tattoo

“A trie will never judge your spelling—but if you don’t mark the end of your words, it’ll never forgive you either.”