[LeetCode] 208. Implement Trie (Prefix Tree)

Implement a trie with insert, search, and startsWith methods.
Note:
You may assume that all inputs are consist of lowercase letters a-z.

Thought process:
Insert: create a new node and add it to the map if current node does not contain current character.
Search: return false if current node does not contain current character, or if the node at the end of searching is not a word.
Startswith: return false if current node does not contain current character.

Solution 1 (Java):

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class Trie {
    class TrieNode {
        TrieNode[] children;
        boolean isWord;
        
        TrieNode() {
            children = new TrieNode[26];
        }
    }
    
    private TrieNode root;

    /** Initialize your data structure here. */
    public Trie() {
        root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        TrieNode current = root;
        for (char c : word.toCharArray()) {
            int index = c - 'a';
            if (current.children[index] == null) {
                current.children[index] = new TrieNode();
            }
            current = current.children[index];
        }
        current.isWord = true;
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode current = root;
        for (char c : word.toCharArray()) {
            int index = c - 'a';
            if (current.children[index] == null) {
                return false;
            }
            current = current.children[index];
        }
        return current.isWord;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        TrieNode current = root;
        for (char c : prefix.toCharArray()) {
            int index = c - 'a';
            if (current.children[index] == null) {
                return false;
            }
            current = current.children[index];
        }
        return true;
    }
}

/**
 * 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);
 */

Given all inputs consist of lowercase letters, we can simplify the hashmap in each trie node to an array of size 26. All three methods require iterating through the input word.

Solution 2 (C++):
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class TrieNode {
public:
    TrieNode* children[26];
    bool isWord;
    
    TrieNode() {
        memset(children, 0, sizeof(children));
        isWord = false;
    }
};

class Trie {
private:
    TrieNode* root;

public:
    /** Initialize your data structure here. */
    Trie() {
        root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    void insert(string word) {
        TrieNode* current = root;
        
        for (int i = 0; i < word.length(); i++) {
            int index = word[i] - 'a';
            if (current->children[index] == NULL) {
                current->children[index] = new TrieNode();
            }
            current = current->children[index];
        }
        current->isWord = true;
    }
    
    /** Returns if the word is in the trie. */
    bool search(string word) {
        TrieNode* current = root;
        
        for (int i = 0; i < word.length(); i++) {
            int index = word[i] - 'a';
            if (current->children[index] == NULL) {
                return false;
            }
            current = current->children[index];
        }
        
        return current->isWord;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) {
        TrieNode* current = root;
        
        for (int i = 0; i < prefix.length(); i++) {
            int index = prefix[i] - 'a';
            if (current->children[index] == NULL) {
                return false;
            }
            current = current->children[index];
        }
        
        return true;
    }
};

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

Time complexity:
All three methods takes O(n), where n is the length of the input string.
Location: Philadelphia, PA, USA

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