208.Implement Trie (Prefix Tree)
===

###### tags: `Medium`,`Trie`,`String`,`Hash Table`

[208. Implement Trie (Prefix Tree)](https://leetcode.com/problems/implement-trie-prefix-tree/)

### 題目描述

A [trie](https://en.wikipedia.org/wiki/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.

### 範例

**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 * 10^4^ calls **in total** will be made to `insert`, `search`, and `startsWith`.

### 解答
#### Python

class Trie:

    def __init__(self):
        self.trie = {}

    def insert(self, word: str) -> None:
        node = self.trie
        for c in word:
            node = node.setdefault(c, {})
        node['$'] = True

    def search(self, word: str) -> bool:
        node = self.trie
        for c in word:
            if not (node := node.get(c)):
                return False
        return '$' in node

    def startsWith(self, prefix: str) -> bool:
        node = self.trie
        for c in prefix:
            if not (node := node.get(c)):
                return False
        return True

> [name=Ron Chen][time=Fri, Mar 17, 2023]

### Reference

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