Log reverse_sublist attempt (solved with hints, optimal) and add flashcards

Author: Brian Elinsky

README.md

@@ -8,7 +8,7 @@ My journey through Elements of Programming Interviews in Python.
 Problems attempted but not yet solved
 
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-☑️☑️☑️☑️⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜
+☑️☑️☑️☑️☑️⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜
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@@ -19,13 +19,13 @@ Problems attempted but not yet solved
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-**24 / 204** (11.8%)
+**25 / 204** (12.3%)
 
 ### Tier 1: Solved 👍
 Problems solved (with or without help)
 
 👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍
-👍⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜
+👍👍⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜
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@@ -36,7 +36,7 @@ Problems solved (with or without help)
 ⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜
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-**21 / 204** (10.3%)
+**22 / 204** (10.8%)
 
 ### Tier 2: Solved Independently 💪
 Problems solved without hints or looking at solutions
@@ -88,12 +88,12 @@ Progress toward priority-based learning goals.
 
 **6 / 16** problems mastered
 
-### 🟠 P1: Attempt All (5/21 - 24%)
+### 🟠 P1: Attempt All (6/21 - 29%)
 
-☑️☑️☑️☑️☑️⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜
+☑️☑️☑️☑️☑️☑️⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜⬜
 ⬜
 
-**5 / 21** problems attempted
+**6 / 21** problems attempted
 
 ### 🟠 P1: Master All (1/21 - 5%)
 
@@ -284,7 +284,7 @@ flowchart TB
 | 4: Primitive Types | 11 | 5 (45%) | 5 (45%) | 3 (27%) | 2 (18%) |
 | 5: Arrays | 20 | 4 (20%) | 3 (15%) | 2 (10%) | 2 (10%) |
 | 6: Strings | 12 | 3 (25%) | 3 (25%) | 1 (8%) | 1 (8%) |
-| 7: Linked Lists | 13 | 1 (8%) | 1 (8%) | 1 (8%) | 1 (8%) |
+| 7: Linked Lists | 13 | 2 (15%) | 2 (15%) | 1 (8%) | 1 (8%) |
 | 8: Stacks and Queues | 9 | 1 (11%) | 1 (11%) | 1 (11%) | 0 (0%) |
 | 9: Binary Trees | 15 | 1 (7%) | 0 (0%) | 0 (0%) | 0 (0%) |
 | 10: Heaps | 6 | 1 (17%) | 1 (17%) | 0 (0%) | 0 (0%) |
@@ -297,7 +297,7 @@ flowchart TB
 | 17: Greedy Algorithms and Invariants | 8 | 1 (12%) | 1 (12%) | 0 (0%) | 0 (0%) |
 | 18: Graphs | 8 | 1 (12%) | 1 (12%) | 0 (0%) | 0 (0%) |
 | 24: Honors Class | 35 | 0 (0%) | 0 (0%) | 0 (0%) | 0 (0%) |
-| **Total** | **204** | **24** | **21** | **11** | **8** |
+| **Total** | **204** | **25** | **22** | **11** | **8** |
 
 ## All Problems
 
@@ -370,7 +370,7 @@ flowchart TB
 | # | Problem | Priority | Status | Best Time |
 |---|---------|----------|--------|-----------|
 | 7.01 | Merge two sorted lists | 🔴 | 🏆 | 3 min |
-| 7.02 | Reverse a single sublist | 🟠 |  |  |
+| 7.02 | Reverse a single sublist | 🟠 | 👍 |  |
 | 7.03 | Test for cyclicity | 🟠 |  |  |
 | 7.04 | Test for overlapping lists---lists are cycle-free | 🟡 |  |  |
 | 7.05 | Test for overlapping lists---lists may have cycles |  |  |  |

problem-questions/07_linked_lists/7.02_reverse_sublist/00_why_dummy_node.md

@@ -0,0 +1,5 @@
+# Why Dummy Node
+
+**Q:** In reverse_sublist, why use a dummy node?
+
+**A:** Handles the case where `start=1`. Without it, we'd have no node before the sublist to anchor to.

problem-questions/07_linked_lists/7.02_reverse_sublist/01_two_pointers.md

@@ -0,0 +1,7 @@
+# Two Pointers for Reversal
+
+**Q:** In reverse_sublist, what two pointers do you need for the reversal, and where do they point?
+
+**A:**
+- `sublist_head`: node BEFORE the sublist (stays fixed)
+- `sublist_iter`: first node OF the sublist (stays on same node, ends up last)

problem-questions/07_linked_lists/7.02_reverse_sublist/02_positioning.md

@@ -0,0 +1,5 @@
+# Positioning Before Sublist
+
+**Q:** To position `sublist_head` at the node before position `start` (1-indexed), how many times do you advance from the dummy node?
+
+**A:** `start - 1` times. If `start=1`, advance 0 times (stay at dummy). If `start=2`, advance 1 time.

problem-questions/07_linked_lists/7.02_reverse_sublist/03_iteration_count.md

@@ -0,0 +1,5 @@
+# Iteration Count
+
+**Q:** To reverse a sublist from position `start` to `finish`, how many iterations of the reversal loop?
+
+**A:** `finish - start` iterations. A sublist of length 3 needs 2 moves to reverse.

problem-questions/07_linked_lists/7.02_reverse_sublist/04_reversal_concept.md

@@ -0,0 +1,5 @@
+# Reversal Concept
+
+**Q:** In reverse_sublist, what does each iteration of the reversal loop do conceptually?
+
+**A:** Takes the node after `sublist_iter` and moves it to the front of the sublist (right after `sublist_head`).

problem-questions/07_linked_lists/7.02_reverse_sublist/05_visual_trace.md

@@ -0,0 +1,9 @@
+# Visual Trace
+
+**Q:** Show the list after each iteration of reversing `1 → [2 → 3 → 4 → 5] → 6` (positions 2-5).
+
+**A:**
+- Initial: `1 → [2 → 3 → 4 → 5] → 6`
+- After iteration 1 (move 3 to front): `1 → [3 → 2 → 4 → 5] → 6`
+- After iteration 2 (move 4 to front): `1 → [4 → 3 → 2 → 5] → 6`
+- After iteration 3 (move 5 to front): `1 → [5 → 4 → 3 → 2] → 6`

problem-questions/07_linked_lists/7.02_reverse_sublist/06_move_node_concept.md

@@ -0,0 +1,8 @@
+# Move Node to Front (Concept)
+
+**Q:** What are the steps to move a node to the front of the sublist (no code)?
+
+**A:**
+1. Save a pointer to the node to move (the one after `sublist_iter`)
+2. Cut it out of the chain
+3. Splice it in at the front (right after `sublist_head`)

problem-questions/07_linked_lists/7.02_reverse_sublist/07_move_node_code.md

@@ -0,0 +1,11 @@
+# Move Node to Front (Code)
+
+**Q:** What are the 4 pointer operations to move `sublist_iter.next` to the front of the sublist?
+
+**A:**
+```python
+node_to_move = sublist_iter.next          # 1. save pointer to node to move
+sublist_iter.next = node_to_move.next     # 2. cut it out of chain
+node_to_move.next = sublist_head.next     # 3. splice: point to current front
+sublist_head.next = node_to_move          # 4. splice: becomes new front
+```

problem-questions/07_linked_lists/7.02_reverse_sublist/08_full_algorithm.md

@@ -0,0 +1,13 @@
+# Full Algorithm
+
+**Q:** Describe the full algorithm for reverse_sublist(L, start, finish).
+
+**A:**
+1. Create dummy node pointing to L
+2. Advance `start - 1` times to position `sublist_head` (node before sublist)
+3. Set `sublist_iter = sublist_head.next` (first node of sublist)
+4. Loop `finish - start` times:
+   - Save pointer to node after `sublist_iter`
+   - Cut it out of chain
+   - Splice it in at front (after `sublist_head`)
+5. Return `dummy.next`