docs(readme): add decision cheat sheet and mental checklist for problem solving

README.md

@@ -14,3 +14,63 @@ Relevant problems for myself
 - 🎨 Guides
 - ⚡ Solutions
 - ♿ Patterns
+
+
+## The Golden Rule (Memorize This)
+
+**Only sort if the problem asks for ORDER or RANKING.**
+
+If the problem asks for:
+- "first"
+- "index"
+- "position"
+- "does it exist"
+- "true/false"
+- "count how many"
+
+→ **DO NOT SORT**. Just use the frequency map and loop.
+
+---
+
+## Decision Cheat Sheet
+
+| What the problem wants                  | Do you need to sort? | What to do instead                  | Example Problem     |
+|-----------------------------------------|----------------------|-------------------------------------|---------------------|
+| Sort characters by how often they appear | YES                 | Freq + Sort                         | LC 451             |
+| Top K most frequent                     | YES                 | Freq + Sort + slice                 | LC 347             |
+| Sort array by frequency                 | YES                 | Freq + Sort                         | LC 1636            |
+| First unique character (index)          | NO                  | Freq + loop original string         | LC 387             |
+| Is it an anagram?                       | NO                  | Freq + check counts                 | LC 242             |
+| How many numbers smaller than current   | Kind of             | Freq + prefix count                 | LC 1365            |
+| Most frequent element                   | NO                  | Freq + find max                     | LC 169             |
+
+---
+
+## Mental Checklist (Say this out loud every time)
+
+1. "Did I count the frequencies?" → Yes
+2. "Does the problem care about the **original order**?"  
+   - Yes → Loop the original string/array and check freq
+   - No  → You can sort
+3. "Does it want an **index** or a **boolean**?"  
+   - Yes → Almost never sort
+4. "Does it want the elements **re-ordered**?"  
+   - Yes → Sort by frequency
+
+---
+
+## Super Short Version to Tattoo in Your Brain
+
+**"If it wants the first / index / true-false → loop original.  
+If it wants sorted / top k / reordered → sort."**
+
+---
+
+## Practice Trigger Words
+
+- "first non-repeating" → Loop original
+- "return its index" → Loop original  
+- "sort by frequency" → Sort
+- "top k frequent" → Sort + slice
+- "valid anagram" → Just check counts
+