docs(docs): add Approach diagrams and Explanation sections to LeetCode problem documentation

apps/docs/content/(hash-table)/208-implement-trie-prefix-tree.mdx

@@ -19,6 +19,44 @@ sidebar:
 - `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.
 
+## Approach
+
+```mermaid
+flowchart TD
+  subgraph state["state — root TrieNode; every node holds a children dict and an endOfWord flag"]
+    R["root"] --- C["... children[c] ..."] --- L["endOfWord = True — a word ends here"]
+  end
+  subgraph insert["insert(word)"]
+    I1["cur = root"] --> I2{"more c in word?"}
+    I2 -- yes --> I3{"c not in cur.children?"}
+    I3 -- yes --> I4["cur.children[c] = TrieNode()"]
+    I4 --> I5["cur = cur.children[c]"]
+    I3 -- no --> I5
+    I5 --> I2
+    I2 -- no --> I6["cur.endOfWord = True"]
+    I6 --> I7(["done"])
+  end
+  subgraph search["search(word)"]
+    S1["cur = root"] --> S2{"more c in word?"}
+    S2 -- yes --> S3{"c not in cur.children?"}
+    S3 -- yes --> S4(["return False"])
+    S3 -- no --> S5["cur = cur.children[c]"]
+    S5 --> S2
+    S2 -- no --> S6(["return cur.endOfWord — the path exists, but is it a whole word?"])
+  end
+  subgraph startsWith["startsWith(prefix)"]
+    P1["cur = root"] --> P2{"more c in prefix?"}
+    P2 -- yes --> P3{"c not in cur.children?"}
+    P3 -- yes --> P4(["return False"])
+    P3 -- no --> P5["cur = cur.children[c]"]
+    P5 --> P2
+    P2 -- no --> P6(["return True — every prefix char had a path"])
+  end
+  state --> insert
+  state --> search
+  state --> startsWith
+```
+
 ## Solution
 
 ```py
@@ -68,3 +106,7 @@ class Trie:
 # param_2 = obj.search(word)
 # param_3 = obj.startsWith(prefix)
 ```
+
+## Explanation
+
+<YouTube id="oobqoCJlHA0" title="Implement Trie (Prefix Tree) - Leetcode 208" />