S0208_implement_trie_prefix_tree

208. Implement Trie (Prefix Tree)

Medium

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 * 104 calls in total will be made to insert, search, and startsWith.

Solution

public class Trie {
    private TrieNode root;
    private bool startWith;

    private class TrieNode {
        // Initialize your data structure here.
        public TrieNode[] children;
        public bool 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[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 bool Search(string word) {
        return Search(word, root, 0);
    }

    private bool Search(string word, TrieNode root, int idx) {
        if (idx == word.Length) {
            startWith = true;
            return root.isWord;
        }
        int index = word[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 bool StartsWith(string prefix) {
        Search(prefix);
        return startWith;
    }
}
/**
 * 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);
}