LeetCode Progress §

Tracking problems practiced with detailed notes on bugs, learnings, and nuances.

Problems §

• 752-open-lock - BFS level tracking, visited timing, Python modulo with negatives

• 279-perfect-squares - Graph modeling insight, BFS for “minimum operations” problems, continue vs break

• 207-course-schedule - Kahn’s algorithm, topological sort, cycle detection via in-degree

• 1971-find-if-path-exists-in-graph - Basic BFS path existence, undirected graph traversal

• 797-all-paths-from-source-to-target - BFS all paths in DAG; path reference copying gotcha

• 116-populating-next-right-pointers-in-each-node - BFS level traversal; O(1) space uses next pointers as linked list (revisit)

• 1091-shortest-path-in-binary-matrix - BFS shortest path; 8-direction grid; mutate grid for O(1) space

• 429-n-ary-tree-level-order-traversal - BFS level order; n-ary just loops over children list

• 210-course-schedule-ii - Kahn’s topological sort; cycle detection via result length check

• 1136-parallel-courses - Topological sort + level counting; min semesters = BFS depth

• 269-alien-dictionary - Hard; extract ordering from adjacent words + topological sort; adjacency direction matters

• 310-minimum-height-trees - ⚠️ REVISIT; peel leaves to find tree center; track remaining nodes not queue size

• 1971-find-if-path-exists-in-graph - DFS existence check, when to backtrack vs shared visited set

• 797-all-paths-from-source-to-target - DFS backtracking, DAG vs general graph visited set requirements

• 1059-all-paths-from-source-lead-to-destination - DFS with visited/verified sets, cycle detection in directed graphs, topo sort alternative

• 133-clone-graph - Graph cloning with dict mapping, add to dict before recursing to handle cycles

• 215-kth-largest-element-in-an-array - Min heap of size k, kth largest at root, O(n log k)

• 1046-last-stone-weight - Max heap via negation, heapify returns None, simulation

• 973-k-closest-points-to-origin - Max heap for k smallest, tuple order matters, no sqrt needed

• 355-design-twitter - ⚠️ REDO - Merge k sorted lists with heap, many implementation bugs

• 621-task-scheduler - ⚠️ REDO - Heap + queue simulation, or math formula, cooldown scheduling

• 323-number-of-connected-components-in-an-undirected-graph - DFS/BFS traversal, mark visited when adding to queue

• 94-binary-tree-inorder-traversal - Recursive vs iterative, nonlocal usage, stack simulates recursion

• 144-binary-tree-preorder-traversal - Push right then left for correct order, simpler than inorder iterative

• 285-inorder-successor-in-bst - BST search from root, track successor when going left, O(H) iterative

• 285-inorder-predecessor-in-bst - Mirror of successor; track predecessor when going right, O(H) iterative

• 173-binary-search-tree-iterator - Controlled inorder with stack; amortized O(1) next(), O(H) space

• 700-search-in-a-binary-search-tree - Basic BST search; iterative O(1) space vs recursive O(H) space

• 701-insert-into-a-binary-search-tree - Insert as leaf; recursive is cleaner, iterative needs parent tracking

• 206-reverse-linked-list - Recursive reversal; head.next.next pointer manipulation, unwinding call stack

• 118-pascals-triangle - Iterative and recursive row building; bucket brigade return pattern

• 119-pascals-triangle-ii - Recursive single-row return; O(rowIndex) space optimization; minimal base cases

• 139-word-break - Recursive consume-from-front; memoize failed suffixes; O(nmk) with cache

• 394-decode-string - Stack with [ boundary marker; prepend when popping to preserve decoded string order

• 208-implement-trie - Trie insert/search/startsWith/delete; count-based deletion; when trie vs hash set

• 119-pascals-triangle-ii - Recursive single-row optimization; only rowIndex==0 base case needed

• 139-word-break - Consume from front with startsWith; memoize failed suffixes; O(nmk)

• custom-unique-elements-across-k-lists - Set of list IDs per num; MapReduce extension with hash partitioning

• 1244-design-a-leaderboard - Sorting vs lazy deletion heap; score accumulation; seen set for dedup

• 78-subsets - Include/exclude backtracking; binary decision tree; 2^n subsets

• 39-combination-sum - Include/exclude with reuse; “how many of each” mental model; prune on sum > target

• 46-permutations - Loop all + seen set; permutations vs subsets pattern; backtrack inside loop

• 90-subsets-ii - Sort + skip duplicates in exclude branch only; pruning prevents redundant paths

• 22-generate-parentheses - Backtrack with open/close counts; 2^(2n) tree pruned to Catalan number

• 79-word-search - Grid backtracking; mark/restore outside loop; O(mn4^L) time

• backtracking-complexity-reference - Framework: choices^depth; cheat sheet for all backtracking problems

• 17-letter-combinations-of-a-phone-number - Digit-to-letter backtracking; O(n * 4^n) time; auxiliary vs total space

• 74-search-a-2d-matrix - Binary search on virtual flat array; mid//cols and mid%cols conversion

• 153-find-minimum-in-rotated-sorted-array - Compare mid vs end; start < end to narrow to one; loop condition choice