From ce2e923019b84c4b874ccde7248c0ade7fa6c4e4 Mon Sep 17 00:00:00 2001
From: Yuto729 <mitomi.yuuto2003@gmail.com>
Date: Wed, 8 Jul 2026 23:50:55 +0900
Subject: [PATCH] solve

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+# 208. Implement Trie (Prefix Tree)
+
+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
+
+## Step1
+
+- lowercase only
+
+```py
+
+class TrieNode:
+    def __init__(self):
+        self.children = {}
+        self.is_end = False
+
+class Trie:
+    def __init__(self):
+        self.root = TrieNode()
+
+    def insert(self, word):
+        node = self.root
+        for c in word:
+            if c in node.children:
+                node = node.children[c]
+                continue
+
+            node.children[c] = TrieNode()
+            node = node.children[c]
+
+        node.is_end = True
+
+    def search(self, word):
+        node = self.root
+        for c in word:
+            if c not in node.children:
+                return False
+
+            node = node.children[c]
+
+        return node.is_end
+
+    def startsWith(self, word):
+        node = self.root
+        for c in word:
+            if c not in node.children:
+                return False
+
+            node = node.children[c]
+
+        return True
+```
+
+AI review
+
+- startsWithの引数はwordというより`prefix`の方がいい
+- searchとstartsWithはロジックがほぼ同じなので、共通メソッドを定義する方が良い
+
+共通メソッドの例
+
+```py
+def _lookup(self, word):
+    node = self.root
+    for c in word:
+        if c not in node.children:
+            return None
+
+        node = node.children[c]
+
+    return node
+```
+
+呼び出す時は、Noneかどうかをチェックする
+
+test
+
+- insert("app") -> search("app") -> search("ap")
+
+insert("app")のトレース
+
+root = Node{}
+
+in loop
+// root = Node{a: Node{}}
+// root = Node{a: Node{p: Node{}}}
+// root = Node{a: Node{p: Node{p: Node{is_end=True}}}}
+
+search("app")のトレース
+node = Node{a: {p: Node{p: Node{is_end=True}}}}
+
+in loop
+// node = Node{p: Node{p: Node{is_end=True}}}}
+// node = Node{p: Node{is_end=True}}}
+// node = Node{is_end=True}}
+node.is_end = TrueなのでTrueを返す
+
+search("ap")のトレース
+node = Node{a: {p: Node{p: Node{is_end=True}}}}
+
+in loop
+// node = Node{p: Node{p: Node{is_end=True}}}}
+// node = Node{p: Node{is_end=True}}}}
+
+node.is_end = FalseなのでFalseを返す
+
+### follow up
+
+- 計算量
+  - 時間計算量 insert, search, startsWith -> O(N)
+  - 空間計算量 O(N \* insertの回数)
+
+- deleteをサポートするとどうなるか？
+
+再帰でdelete関数を書くのがいい。再帰関数は「部下に、入力のwordの次の文字以降を部分木から削除してもらい、部下自身を削除しても良いかどうかを返してもらう」
+削除してよければ削除をする。自身を削除して良い条件は、全ての子も削除してよく、
+
+```py
+def delete(self, word):
+    def _delete(node, index):
+        if index == len(word):
+            if not node.is_end:
+                return False # そもそも登録されていない単語
+
+            node.is_end = False
+            # 子がなければ、このノード自体を親から削除して良い
+            return len(node.children) == 0
+
+        c = word[index]
+        if c not in node.children:
+            return False # 単語が存在しない
+
+        child = node.children[c]
+        should_delete_child = _delete(child, index + 1)
+        if should_delete_child:
+            del node.children[c]
+            # 自分自身も子がなく、単語の終端でもないなら親から削除して良い
+            return len(node.children) == 0 and not node.is_end
+
+        return False
+
+    _delete(self.root, 0)
+```
+
+## Step3
+
+```py
+class TrieNode:
+    def __init__(self):
+        self.children = {}
+        self.is_end = False
+
+class Trie:
+
+    def __init__(self):
+        self.root = TrieNode()
+
+    def insert(self, word: str) -> None:
+        node = self.root
+        for c in word:
+            if c not in node.children:
+                node.children[c] = TrieNode()
+
+            node = node.children[c]
+
+        node.is_end = True
+
+    def search(self, word: str) -> bool:
+        last_character_node = self._lookup(word)
+        if last_character_node is None:
+            return False
+
+        return last_character_node.is_end
+
+    def startsWith(self, prefix: str) -> bool:
+        last_character_node = self._lookup(prefix)
+        return last_character_node is not None
+
+    def _lookup(self, word: str) -> TrieNode | None:
+        node = self.root
+        for c in word:
+            if c not in node.children:
+                return None
+
+            node = node.children[c]
+
+        return node
+```