Update 'list.md'

list.md

@@ -5,107 +5,130 @@ These exercises will help you get really good at using lists in Python. You'll p
 
 ---
 
-## Exercise 1
+## PYEKX-6280ca9a
 
-**Flatten a Nested List**
+**Flatten a Deeply Nested List**
 
 ### Problem
-
-Write a function `flatten_list(nested_list)` that takes a list which may contain nested lists of any depth, and returns a single, flat list.
+Write a function `flatten_list(nested_list)` that flattens an arbitrarily nested list into a single-level list.
 
 ### Example
-
 ```python
 flatten_list([1, [2, 3], [4, [5, 6], 7]])
 # Output: [1, 2, 3, 4, 5, 6, 7]
 ```
 
-### Concepts
+### Why? (*Why it matters*)
+Flattening is common in:
+- ETL pipelines
+- JSON parsing
+- Recursive DOM traversal
 
-- Recursion
-- `isinstance` type checking
-- `extend` vs `append`
+### Pro Constraints
+- Must support arbitrary levels of nesting.
+- Must not mutate the input list.
+- Use recursion or a stack-based approach.
+- Bonus: Implement both recursive and iterative versions.
 
 ---
 
-## Exercise 2
+## PYEKX-1d45ec3e
 
 **Group Consecutive Duplicates**
 
 ### Problem
-
-Write a function `group_consecutive(lst)` that groups consecutive identical elements into sublists.
+Write a function `group_consecutive(lst)` that groups **only consecutive** duplicate elements into sublists.
 
 ### Example
-
 ```python
 group_consecutive([1, 1, 2, 2, 2, 3, 1, 1])
 # Output: [[1, 1], [2, 2, 2], [3], [1, 1]]
 ```
 
-### Concepts
+### Why?
+This mimics:
+- Run-length encoding
+- Log event grouping
+- Real-time batching by last-seen state
 
-- Iteration with state tracking
-- Temporary sublists
-- `for` loops with conditionals
+### Pro Constraints
+- Avoid using third-party libraries.
+- Maintain original order.
+- Think about how to handle empty input and non-integer elements.
 
 ---
 
-## Exercise 3
+## PYEKX-3c65900f
 
-**Rotate List k Times**
+**Rotate List (Bidirectional) k Times**
 
 ### Problem
-
 Write a function `rotate_list(lst, k)` that rotates the list **right** by `k` steps.
-
-> BONUS: Support negative `k` to rotate left.
+> Support negative `k` to rotate left.
 
 ### Example
-
 ```python
 rotate_list([1, 2, 3, 4, 5], 2)
 # Output: [4, 5, 1, 2, 3]
+
+rotate_list([1, 2, 3, 4, 5], -2)
+# Output: [3, 4, 5, 1, 2]
 ```
 
-### Concepts
+### Why?
+- Common in **buffer management, scheduling, circular data** structures.
+- Reinforces modular math and slicing mastery.
 
-- Identify concepts yourself.
+### Pro Constraints
+- Solve it using slicing.
+- Then try using only loops - no slicing.
+- Handle large `k` values: `rotate_list([1,2,3], 100)`.
 
 ---
 
-## Exercise 4
+## PYEKX-6a7bc172
 
-**List Difference by Value and Order**
+**List Difference (Ordered & Counted)**
 
 ### Problem
-
-Write a function `diff_lists(a, b)` that removes **each occurrence** in `b` from `a`, respecting the number of times elements occur.
+Write a function `diff_lists(a, b)` that removes one matching element in `b` for each occurrence found in `a`, preserving order.
 
 ### Example
-
 ```python
 diff_lists([1, 2, 2, 3, 4], [2, 4])
 # Output: [1, 2, 3]
 ```
 
-> One `2` and the `4` are removed. The second `2` remains.
+### Why?
+- Models real-world inventory adjustments.
+- Used in symmetric difference, record purging, filtering with quantity.
+
+### Pro Constraints
+- Do not use `collections.Counter`.
+- Optimize for readability and minimal mutations.
+- Bonus: Add a flag to toggle whether duplicates in b remove all vs one.
+
 
-### Concepts
-- Element-by-element removal
-- List copying
-- `in` with deletion logic
 
 ---
 
-## ✅ Pro Tips
-
+## Pro Tips
 - Try solving without using libraries.
 - Then try versions using `collections.Counter`, `itertools`, or `functools` for practice.
-
 - Write unit tests for edge cases:
   - Empty lists
   - Deep nesting
   - Large `k` values
   - Negative numbers
+  ```python
+    def test_flatten():
+        assert flatten_list([1, [2], [[3]]]) == [1, 2, 3]
+        assert flatten_list([]) == []
+        assert flatten_list([[], [[], [[]]]]) == []
+
+    def test_rotate():
+        assert rotate_list([1,2,3], 0) == [1,2,3]
+        assert rotate_list([1,2,3], 3) == [1,2,3]
+        assert rotate_list([1,2,3], -1) == [2,3,1]
+  ```