208. Implement Trie (Prefix Tree)

Description of Problem

A trie (pronounced as "try") or prefix tree is a tree data structure used to efficiently store and retrieve keys in a dataset of strings. There are various applications of this data structure, such as autocomplete and spellchecker.

Implement the Trie class:

Trie() Initializes the trie object.
void insert(String word) Inserts the string word into the trie.
boolean search(String word) Returns true if the string word is in the trie (i.e., was inserted before), and false otherwise.
boolean startsWith(String prefix) Returns true if there is a previously inserted string word that has the prefix prefix, and false otherwise.

Example 1:

Input
["Trie", "insert", "search", "search", "startsWith", "insert", "search"]
[[], ["apple"], ["apple"], ["app"], ["app"], ["app"], ["app"]]
Output
[null, null, true, false, true, null, true]

Explanation
Trie trie = new Trie();
trie.insert("apple");
trie.search("apple");   // return True
trie.search("app");     // return False
trie.startsWith("app"); // return True
trie.insert("app");
trie.search("app");     // return True

Constraints:

1 <= word.length, prefix.length <= 2000
word and prefix consist only of lowercase English letters.
At most 3 * 10^4 calls in total will be made to insert, search, and startsWith.

Solution 1 - using HashMap with Lazy Initialisation

Tags: Trie

Code

import java.util.HashMap;
import java.util.Map;

class Trie {
    private Map<Character,Trie> children;
    private boolean isLastChar;

    public Trie() {
        this.children = null;
        this.isLastChar = false;
    }

    public Trie(boolean isLastChar) {
        this.children = null;
        this.isLastChar = isLastChar;
    }

    public boolean isLastChar(){
        return this.isLastChar;
    }

    public void setIsLastChar(boolean isLastChar){
        this.isLastChar = isLastChar;
    }
    
    public void insert(String word) {
       Trie curr = this;
       int n = word.length();
        for (int i = 0; curr != null && i < n; i++){
            if (curr.children == null){
                curr.children = new HashMap();            
            } 

            Trie next = curr.children.get(word.charAt(i));
            if ( next == null ){
                Trie next_children = new Trie(i == n - 1 ? true: false);
                curr.children.put(word.charAt(i), next_children);
                curr = next_children;
            }else {
                if (i == n - 1) {next.setIsLastChar(true);}
                curr = next;
            }
        }
    }
    
    public boolean search(String word) {
        Trie curr = this;
        int n = word.length();
        for (int i = 0; curr != null && i < n; i++){
            Trie next = curr.children != null ? curr.children.get(word.charAt(i)) : null;
            if ( next == null ){
                return false;
            }else {
                curr = next;
            }
        }
        return curr != null ? curr.isLastChar() : false;
    }
    
    public boolean startsWith(String prefix) {
        Trie curr = this;
        int n = prefix.length();
        for (int i = 0; i < n; i++){
            Trie next = curr.children != null ? curr.children.get(prefix.charAt(i)) : null;
            if ( next == null ){
                return false;
            }else {
                curr = next;
            }
        }
        return curr != null ? true : false;
    }
}

/**
 * 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 2 - using Array

Tags: Trie

Code

class Trie {
    private Trie[] children;
    private boolean isLastChar;

    public Trie() {
        this.children = new Trie[26];
        this.isLastChar = false;
    }

    public Trie(boolean isLastChar) {
        this.children = new Trie[26];
        this.isLastChar = isLastChar;
    }

    public boolean isLastChar(){
        return this.isLastChar;
    }

    public void setIsLastChar(boolean isLastChar){
        this.isLastChar = isLastChar;
    }
    
    public void insert(String word) {
       Trie curr = this;
       int n = word.length();
        for (int i = 0; curr != null && i < n; i++){
            Trie next = curr.children[word.charAt(i) - 'a'];
            if ( next == null ){
                Trie next_children = new Trie(i == n - 1 ? true: false);
                curr.children[word.charAt(i) - 'a'] = next_children;
                curr = next_children;
            }else {
                if (i == n - 1) {next.setIsLastChar(true);}
                curr = next;
            }
        }
    }
    
    public boolean search(String word) {
        Trie curr = this;
        int n = word.length();
        for (int i = 0; curr != null && i < n; i++){
            Trie next = curr.children[word.charAt(i) - 'a'];
            if ( next == null ){
                return false;
            }else {
                curr = next;
            }
        }
        return curr != null ? curr.isLastChar() : false;
    }
    
    public boolean startsWith(String prefix) {
        Trie curr = this;
        int n = prefix.length();
        for (int i = 0; i < n; i++){
            Trie next = curr.children[prefix.charAt(i) - 'a'];
            if ( next == null ){
                return false;
            }else {
                curr = next;
            }
        }
        return curr != null ? true : false;
    }
}

/**
 * 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 3 - using Array with Lazy initialisation

Tags: Trie

It does not consume less RAM however.

Code

class Trie {
    private Trie[] children;
    private boolean isLastChar;

    public Trie() {
        this.children = null;
        this.isLastChar = false;
    }

    public Trie(boolean isLastChar) {
        this.children = null;
        this.isLastChar = isLastChar;
    }

    public boolean isLastChar(){
        return this.isLastChar;
    }

    public void setIsLastChar(boolean isLastChar){
        this.isLastChar = isLastChar;
    }
    
    public void insert(String word) {
       Trie curr = this;
       int n = word.length();
        for (int i = 0; curr != null && i < n; i++){

            if (curr.children == null) {
                curr.children = new Trie[26];
            }

            Trie next = curr.children == null ? null : curr.children[word.charAt(i) - 'a'];
            if ( next == null ){
                Trie next_children = new Trie(i == n - 1 ? true: false);
                curr.children[word.charAt(i) - 'a'] = next_children;
                curr = next_children;
            }else {
                if (i == n - 1) {next.setIsLastChar(true);}
                curr = next;
            }
        }
    }
    
    public boolean search(String word) {
        Trie curr = this;
        int n = word.length();
        for (int i = 0; curr != null && i < n; i++){
            Trie next = curr.children == null ? null : curr.children[word.charAt(i) - 'a'];
            if ( next == null ){
                return false;
            }else {
                curr = next;
            }
        }
        return curr != null ? curr.isLastChar() : false;
    }
    
    public boolean startsWith(String prefix) {
        Trie curr = this;
        int n = prefix.length();
        for (int i = 0; i < n; i++){
            Trie next = curr.children == null ? null : curr.children[prefix.charAt(i) - 'a'];
            if ( next == null ){
                return false;
            }else {
                curr = next;
            }
        }
        return curr != null ? true : false;
    }
}

/**
 * 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);
 */

Leetbook