README_EN.md

208. Implement Trie (Prefix Tree)

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Description

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.

Solutions

Python3

class Trie:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.children = [None] * 26
        self.is_end = False

    def insert(self, word: str) -> None:
        """
        Inserts a word into the trie.
        """
        node = self
        for c in word:
            index = ord(c) - ord("a")
            if node.children[index] is None:
                node.children[index] = Trie()
            node = node.children[index]
        node.is_end = True

    def search(self, word: str) -> bool:
        """
        Returns if the word is in the trie.
        """
        node = self._search_prefix(word)
        return node is not None and node.is_end

    def startsWith(self, prefix: str) -> bool:
        """
        Returns if there is any word in the trie that starts with the given prefix.
        """
        node = self._search_prefix(prefix)
        return node is not None

    def _search_prefix(self, prefix: str):
        node = self
        for c in prefix:
            index = ord(c) - ord("a")
            if node.children[index] is None:
                return None
            node = node.children[index]
        return node

# Your Trie object will be instantiated and called as such:
# obj = Trie()
# obj.insert(word)
# param_2 = obj.search(word)
# param_3 = obj.startsWith(prefix)

Java

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

    /** Initialize your data structure here. */
    public Trie() {
        children = new Trie[26];
        isEnd = false;
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        Trie node = this;
        for (int i = 0; i < word.length(); ++i) {
            char c = word.charAt(i);
            int index = c - 'a';
            if (node.children[index] == null) {
                node.children[index] = new Trie();
            }
            node = node.children[index];
        }
        node.isEnd = true;
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        Trie node = searchPrefix(word);
        return node != null && node.isEnd;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        Trie node = searchPrefix(prefix);
        return node != null;
    }

    private Trie searchPrefix(String prefix) {
        Trie node = this;
        for (int i = 0; i < prefix.length(); ++i) {
            char c = prefix.charAt(i);
            int index = c - 'a';
            if (node.children[index] == null) {
                return null;
            }
            node = node.children[index];
        }
        return node;
    }
}

/**
 * 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);
 */

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