Implement Trie (Prefix Tree)

Medium Trie Design

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2020-05-14

208. Implement Trie (Prefix Tree)

Question:

Implement a trie with insert, search, and startsWith methods.

Example:

Trie trie = new Trie();

trie.insert("apple");
trie.search("apple");   // returns true
trie.search("app");     // returns false
trie.startsWith("app"); // returns true
trie.insert("app");   
trie.search("app");     // returns true

Note:

You may assume that all inputs are consist of lowercase letters a-z.
All inputs are guaranteed to be non-empty strings.

Solution:

Create a data structure for TrieNode with a children array and boolean hasWord value. If reach to the last character of the word, set the hasWord to true. Use char - ‘a’ to decide the index of the children.

// Data Structure for TrieNode
class TrieNode {
    public TrieNode[] children;
    public boolean hasWord;

    public TrieNode() {
        children = new TrieNode[26];
        hasWord = false;
    }
}

class Trie {
    
    TrieNode root;
    
    /** Initialize your data structure here. */
    public Trie() {
        root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        TrieNode curr = root;
        for (char c : word.toCharArray()) {
            if (curr.children[c - 'a'] == null) {
                curr.children[c - 'a'] = new TrieNode();
            }
            curr = curr.children[c - 'a'];
        }
        curr.hasWord = true;
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode lastnode = searchHelper(word);
        return lastnode != null && lastnode.hasWord;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        TrieNode lastnode = searchHelper(prefix);
        return lastnode != null;
    }
    
    // Helper to return the last TrieNode. If cannot find one, return null. 
    private TrieNode searchHelper(String word) {
        TrieNode curr = root;
        for (char c : word.toCharArray()) {
            if (curr.children[c - 'a'] == null) {
                return null;
            }
            curr = curr.children[c - 'a'];
        }
        return curr;
    }
}

/**
 * Your Trie object will be instantiated and called as such:
 * Trie obj = new Trie();
 * obj.insert(word);
 * boolean param_2 = obj.search(word);
 * boolean param_3 = obj.startsWith(prefix);
 */