LeetCode Notebook

Raw working notebook - LeetCode traces, interview framing, BFS walkthroughs. Full problem tracker on Notion.

Interview Framing: Odd-Even Linked List

How to explain the odd-even linked list in an interview:

“I used a two-pointer strategy - one for odd-indexed nodes, one for even. I treat the list as two interleaved sublists and rewire .next pointers to separate them.

The critical insight: updating .next alone is not enough - I also need to advance the tail pointer. Once I update odd.next, I move odd = odd.next so the tail stays accurate for future rewiring.

After separation, I stitch the odd list’s end to the saved even-list head. In-place, O(1) space, O(n) time.”

Key phrasings that signal depth:

• “I reused the existing node structure instead of allocating new nodes.”
• “Safe pointer progression - modifying .next and moving the active pointer in tandem.”
• “It’s a pointer-surgery problem with clear sequencing logic.”

BFS Walkthrough: 909. Snakes and Ladders

6x6 board (positions 1-36) with ladders (2->15, 14->35) and a snake (17->13). Goal: reach 36 in minimum dice rolls.

Why BFS: each roll branches into 6 next states. BFS explores all states at depth d before d+1, so the first path to 36 is guaranteed shortest.

Board as flat array (0-indexed, position 0 unused):

1[ 0, -1, 15, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 35, -1, -1, 13, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1 ]

Critical path: 1 -> 2(ladder->15) -> … -> 13(via snake) -> 14(ladder->35) -> 36

 1=== Move 1 ===
 2Queue: [1]
 3Processing position: 1
 4  Enqueued: 15 (pos 2 -> ladder)
 5  Enqueued: 3, 4, 5, 6, 7
 6
 7=== Move 2 ===
 8Queue: [15, 3, 4, 5, 6, 7]
 9Processing position: 15 -> enqueues 16, 13(snake from 17), 18, 19, 20, 21
10Processing positions: 3-7 -> enqueues 8, 9, 10, 11, 12
11  (most neighbours already visited)
12
13=== Move 3 ===
14Queue: [16, 13, 18, 19, 20, 21, 8, 9, 10, 11, 12]
15Processing position: 13 -> enqueues 35 (pos 14 -> ladder!), rest visited
16Processing others -> enqueues 22-27
17  (critical: position 14's ladder to 35 discovered here)
18
19=== Move 4 ===
20Queue: [22, 35, 23, 24, 25, 26, 27]
21Processing position: 35 -> enqueues 36
22Processing others -> enqueues 28-33
23
24=== Move 5 ===
25Queue: [28, 36, 29, 30, 31, 32, 33]
26Processing position: 36
27  Reached end! Total moves: 4

Answer: 4 moves. Position 36 is enqueued at BFS depth 4 (Move 4). Move 5 dequeues and confirms.