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208. Implement Trie (Prefix Tree)

Calf

Problem

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.

https://leetcode.com/problems/implement-trie-prefix-tree/

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

cpp 
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.

Test Cases

python 
class Trie:

    def __init__(self):


    def insert(self, word: str) -> None:


    def search(self, word: str) -> bool:


    def startsWith(self, prefix: str) -> bool:



# 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)
solution_test.py下载完整源码
py 
import pytest

from solution import Trie


null = None
true = True
false = False


@pytest.mark.parametrize('actions, params, expects', [
    (
        ["Trie", "insert", "search", "search", "startsWith", "insert", "search"],
        [[], ["apple"], ["apple"], ["app"], ["app"], ["app"], ["app"]],
        [null, null, true, false, true, null, true]
    ),
    (
        ["Trie","search"],
        [[],["a"]],
        [null,false]
    ),
])
class Test:
    def test_solution(self, actions, params, expects):
        trie = None
        for action, args, expected in zip(actions, params, expects):
            if action == 'Trie':
                trie = Trie()
            else:
                assert getattr(trie, action)(*args) == expected

Thoughts

就是个树形结构。每个节点保存一个哈希表，key 是字符，value 是下一层节点。为了区分是否完整的单词，最后一个节点的哈希表里加一个特殊的终止字符作为标记。

Code

solution.py下载完整源码
py 
class Trie:
    def __init__(self):
        self._root: dict[str, dict] = {}

    def insert(self, word: str) -> None:
        node = self._root
        for c in word:
            if c not in node:
                node[c] = {}
            node = node[c]

        node['.'] = None # End of a word.

    def search(self, word: str) -> bool:
        node = self._find(word)
        return node is not None and '.' in node

    def startsWith(self, prefix: str) -> bool:
        return self._find(prefix) is not None

    def _find(self, word: str) -> dict | None:
        node = self._root
        for c in word:
            if c not in node:
                return None
            node = node[c]

        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)

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