208. Implement Trie (Prefix Tree)

Problem

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 (Java)

class Trie {

    private TrieNode root;
    private boolean startWith;

    private static class TrieNode {
        // Initialize your data structure here.
        public TrieNode[] children;
        public boolean isWord;

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

    public Trie() {
        root = new TrieNode();
    }

    // Inserts a word into the trie.
    public void insert(String word) {
        insert(word, root, 0);
    }

    private void insert(String word, TrieNode root, int idx) {
        if (idx == word.length()) {
            root.isWord = true;
            return;
        }
        int index = word.charAt(idx) - 'a';
        if (root.children[index] == null) {
            root.children[index] = new TrieNode();
        }
        insert(word, root.children[index], idx + 1);
    }

    // Returns if the word is in the trie.
    public boolean search(String word) {
        return search(word, root, 0);
    }

    public boolean search(String word, TrieNode root, int idx) {
        if (idx == word.length()) {
            startWith = true;
            return root.isWord;
        }
        int index = word.charAt(idx) - 'a';
        if (root.children[index] == null) {
            startWith = false;
            return false;
        }
        return search(word, root.children[index], idx + 1);
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    public boolean startsWith(String prefix) {
        search(prefix);
        return startWith;
    }
}

/**
 * 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);
 */

Solution (Javascript)

var Node = function () {
  this.children = {};
  this.isWord = false;
};

/**
 * Initialize your data structure here.
 */
var Trie = function() {
  this.root = new Node();
};

/**
 * Inserts a word into the trie. 
 * @param {string} word
 * @return {void}
 */
Trie.prototype.insert = function(word) {
  var len = word.length;
  var node = this.root;
  var char = 0;
  for (var i = 0; i < len; i++) {
    char = word[i];
    if (!node[char]) node[char] = new Node();
    node = node[char];
  }
  node.isWord = true;
};

/**
 * Returns if the word is in the trie. 
 * @param {string} word
 * @return {boolean}
 */
Trie.prototype.search = function(word) {
  var len = word.length;
  var node = this.root;
  var char = 0;
  for (var i = 0; i < len; i++) {
    char = word[i];
    if (!node[char]) return false;
    node = node[char];
  }
  return node.isWord;
};

/**
 * Returns if there is any word in the trie that starts with the given prefix. 
 * @param {string} prefix
 * @return {boolean}
 */
Trie.prototype.startsWith = function(prefix) {
  var len = prefix.length;
  var node = this.root;
  var char = 0;
  for (var i = 0; i < len; i++) {
    char = prefix[i];
    if (!node[char]) return false;
    node = node[char];
  }
  return true;
};

/** 
 * Your Trie object will be instantiated and called as such:
 * var obj = Object.create(Trie).createNew()
 * obj.insert(word)
 * var param_2 = obj.search(word)
 * var param_3 = obj.startsWith(prefix)
 */

Explain:

nope.

Complexity:

• Time complexity : O(h).
• Space complexity : O(h).