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

apps/docs/content/(array)/271-encode-and-decode-strings.mdx

@@ -27,6 +27,27 @@ Could you write a generalized algorithm to work on any possible set of character
 - `0 <= strs[i].length <= 200`
 - `strs[i]` contains any possible characters out of 256 valid ASCII characters.
 
+## Approach
+
+::::warning
+The diagram traces the code below literally. That code does not solve the problem: both `encode` and `decode` have only a docstring for a body, so each returns `None` and `codec.decode(codec.encode(strs))` yields `None` instead of `strs`.
+::::
+
+```mermaid
+flowchart TD
+  subgraph state["state — Codec holds no fields"]
+    X["Codec() — nothing is stored between encode and decode"]
+  end
+  subgraph encode["encode(strs)"]
+    E1["docstring only — no statement runs"] --> E2(["return None"])
+  end
+  subgraph decode["decode(s)"]
+    D1["docstring only — no statement runs"] --> D2(["return None"])
+  end
+  state --> encode
+  state --> decode
+```
+
 ## Solution
 
 ```py
@@ -46,3 +67,7 @@ class Codec:
 # codec = Codec()
 # codec.decode(codec.encode(strs))
 ```
+
+## Explanation
+
+<YouTube id="B1k_sxOSgv8" title="Encode and Decode Strings - Leetcode 271 - Python" />