README_EN.md

1804. Implement Trie II (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.
• int countWordsEqualTo(String word) Returns the number of instances of the string word in the trie.
• int countWordsStartingWith(String prefix) Returns the number of strings in the trie that have the string prefix as a prefix.
• void erase(String word) Erases the string word from the trie.

 

Example 1:

Input
["Trie", "insert", "insert", "countWordsEqualTo", "countWordsStartingWith", "erase", "countWordsEqualTo", "countWordsStartingWith", "erase", "countWordsStartingWith"]
[[], ["apple"], ["apple"], ["apple"], ["app"], ["apple"], ["apple"], ["app"], ["apple"], ["app"]]
Output
[null, null, null, 2, 2, null, 1, 1, null, 0]

Explanation
Trie trie = new Trie();
trie.insert("apple");               // Inserts "apple".
trie.insert("apple");               // Inserts another "apple".
trie.countWordsEqualTo("apple");    // There are two instances of "apple" so return 2.
trie.countWordsStartingWith("app"); // "app" is a prefix of "apple" so return 2.
trie.erase("apple");                // Erases one "apple".
trie.countWordsEqualTo("apple");    // Now there is only one instance of "apple" so return 1.
trie.countWordsStartingWith("app"); // return 1
trie.erase("apple");                // Erases "apple". Now the trie is empty.
trie.countWordsStartingWith("app"); // return 0

 

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, countWordsEqualTo, countWordsStartingWith, and erase.
• It is guaranteed that for any function call to erase, the string word will exist in the trie.

Solutions

Python3

class Trie:

    def __init__(self):
        self.children = [None] * 26
        self.count = 0
        self.pre_count = 0

    def insert(self, word: str) -> None:
        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.pre_count += 1
        node.count += 1

    def countWordsEqualTo(self, word: str) -> int:
        node = self._search_prefix(word)
        return 0 if node is None else node.count

    def countWordsStartingWith(self, prefix: str) -> int:
        node = self._search_prefix(prefix)
        return 0 if node is None else node.pre_count

    def erase(self, word: str) -> None:
        node = self
        for c in word:
            index = ord(c) - ord('a')
            node = node.children[index]
            node.pre_count -= 1
        node.count -= 1

    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.countWordsEqualTo(word)
# param_3 = obj.countWordsStartingWith(prefix)
# obj.erase(word)

Java

class Trie {
    private Trie[] children;
    private int count;
    private int preCount;

    public Trie() {
        children = new Trie[26];
        count = 0;
        preCount = 0;
    }

    public void insert(String word) {
        Trie node = this;
        for (int i = 0; i < word.length(); ++i) {
            int index = word.charAt(i) - 'a';
            if (node.children[index] == null) {
                node.children[index] = new Trie();
            }
            node = node.children[index];
            node.preCount += 1;
        }
        node.count += 1;
    }

    public int countWordsEqualTo(String word) {
        Trie node = searchPrefix(word);
        return node == null ? 0 : node.count;
    }

    public int countWordsStartingWith(String prefix) {
        Trie node = searchPrefix(prefix);
        return node == null ? 0 : node.preCount;
    }

    public void erase(String word) {
        Trie node = this;
        for (int i = 0; i < word.length(); ++i) {
            int index = word.charAt(i) - 'a';
            node = node.children[index];
            node.preCount -= 1;
        }
        node.count -= 1;
    }

    private Trie searchPrefix(String prefix) {
        Trie node = this;
        for (int i = 0; i < prefix.length(); ++i) {
            int index = prefix.charAt(i) - '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);
 * int param_2 = obj.countWordsEqualTo(word);
 * int param_3 = obj.countWordsStartingWith(prefix);
 * obj.erase(word);
 */

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