LeetCode-in-All

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

class Trie {
    private val root: TrieNode
    private var startWith = false

    class TrieNode {
        // Initialize your data structure here.
        var children: Array<TrieNode?>
        var isWord = false

        init {
            children = arrayOfNulls(26)
        }
    }

    // Inserts a word into the trie.
    fun insert(word: String) {
        insert(word, root, 0)
    }

    private fun insert(word: String, root: TrieNode?, idx: Int) {
        if (idx == word.length) {
            root!!.isWord = true
            return
        }
        val index = word[idx] - 'a'
        if (root!!.children[index] == null) {
            root.children[index] = TrieNode()
        }
        insert(word, root.children[index], idx + 1)
    }

    // Returns if the word is in the trie.
    @JvmOverloads
    fun search(word: String, root: TrieNode? = this.root, idx: Int = 0): Boolean {
        if (idx == word.length) {
            startWith = true
            return root!!.isWord
        }
        val 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.
    fun startsWith(prefix: String): Boolean {
        search(prefix)
        return startWith
    }

    init {
        root = TrieNode()
    }
}

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