#Problem index

Every problem from the pattern pages, in one place. Roughly 110 problems — which is deliberate: a hundred problems you can re-derive beats three hundred you have seen.

#How to track this

Not with ticks. Ticks measure exposure; interviews measure recall.

Keep four columns per problem:

Column	Meaning
Solved	Got it inside the 25-minute box, unaided
Help level	0 = unaided … 6 = read the full solution (see how to practise)
Day 7	Re-derived a week later, from a blank file
Day 30	Re-derived a month later

The number that predicts interview performance is the day-7 column. A problem solved once and never revisited is not knowledge, it is a memory of having been told something.

A workable minimum: a spreadsheet with problem, pattern, date, help_level, day7, day30. Sort by help_level descending to find what to revisit.

Interactive simulation — needs JavaScript.

#The core 40

If time is short, these forty cover the highest-frequency ground. Marked ⭐ are the ones most likely to appear verbatim.

#	Problem	LC	Pattern
1	⭐ Two Sum	1	Hashing
2	Contains Duplicate	217	Hashing
3	Valid Anagram	242	Hashing
4	⭐ Group Anagrams	49	Hashing
5	Top K Frequent Elements	347	Hashing / heap
6	⭐ Subarray Sum Equals K	560	Prefix sum
7	Longest Consecutive Sequence	128	Hashing
8	Valid Palindrome	125	Two pointers
9	Two Sum II	167	Two pointers
10	⭐ 3Sum	15	Two pointers
11	⭐ Container With Most Water	11	Two pointers
12	Trapping Rain Water	42	Two pointers
13	⭐ Longest Substring Without Repeating	3	Sliding window
14	Longest Repeating Character Replacement	424	Sliding window
15	Permutation in String	567	Sliding window
16	Minimum Size Subarray Sum	209	Sliding window
17	⭐ Minimum Window Substring	76	Sliding window
18	Sliding Window Maximum	239	Deque
19	⭐ Valid Parentheses	20	Stack
20	Min Stack	155	Stack
21	⭐ Daily Temperatures	739	Monotonic stack
22	Largest Rectangle in Histogram	84	Monotonic stack
23	⭐ Binary Search	704	Binary search
24	⭐ Search in Rotated Sorted Array	33	Binary search
25	Find Minimum in Rotated Sorted Array	153	Binary search
26	⭐ Koko Eating Bananas	875	Search on the answer
27	Split Array Largest Sum	410	Search on the answer
28	⭐ Maximum Depth of Binary Tree	104	Trees
29	⭐ Binary Tree Level Order Traversal	102	Trees / BFS
30	⭐ Validate BST	98	Trees
31	⭐ Lowest Common Ancestor	236	Trees
32	Binary Tree Maximum Path Sum	124	Trees
33	⭐ Number of Islands	200	Graphs
34	⭐ Rotting Oranges	994	Multi-source BFS
35	⭐ Course Schedule	207	Topological sort
36	Pacific Atlantic Water Flow	417	Graphs
37	⭐ Kth Largest Element	215	Heap
38	Merge k Sorted Lists	23	Heap
39	⭐ Merge Intervals	56	Intervals
40	⭐ Meeting Rooms II	253	Intervals / heap

The 21 starred problems are the ones I would guarantee you see across a handful of loops. If you can do only twenty things, do those.

#By pattern

#Hashing → [page](hashing.html)

Level	Problems
Easy	Two Sum (1) · Contains Duplicate (217) · Valid Anagram (242) · Majority Element (169)
Medium	Group Anagrams (49) · Top K Frequent (347) · Subarray Sum Equals K (560) · Longest Consecutive (128) · Contiguous Array (525) · 4Sum II (454)
Hard	First Missing Positive (41) · LRU Cache (146)

#Two pointers → [page](two-pointers.html)

Level	Problems
Easy	Valid Palindrome (125) · Two Sum II (167) · Remove Duplicates (26) · Merge Sorted Array (88)
Medium	3Sum (15) · 3Sum Closest (16) · Container With Most Water (11) · Sort Colors (75) · Linked List Cycle II (142) · 4Sum (18) · Boats to Save People (881)
Hard	Trapping Rain Water (42)

#Sliding window → [page](sliding-window.html)

Level	Problems
Easy	Maximum Average Subarray (643) · Contains Duplicate II (219)
Medium	Longest Substring Without Repeating (3) · Longest Repeating Char Replacement (424) · Permutation in String (567) · Minimum Size Subarray Sum (209) · Fruit Into Baskets (904) · Max Consecutive Ones III (1004) · At Most K Distinct (340) · Subarrays with K Different (992)
Hard	Minimum Window Substring (76) · Sliding Window Maximum (239)

#Stack → [page](stack.html)

Level	Problems
Easy	Valid Parentheses (20) · Min Stack (155) · Baseball Game (682) · Remove Adjacent Duplicates (1047)
Medium	Daily Temperatures (739) · Next Greater Element II (503) · Evaluate RPN (150) · Asteroid Collision (735) · Simplify Path (71) · Decode String (394) · Car Fleet (853) · Online Stock Span (901)
Hard	Largest Rectangle (84) · Maximal Rectangle (85)

#Binary search → [page](binary-search.html)

Level	Problems
Easy	Binary Search (704) · Search Insert Position (35) · First Bad Version (278)
Medium	First and Last Position (34) · Search in Rotated Array (33) · Find Minimum in Rotated (153) · Koko Eating Bananas (875) · Capacity To Ship (1011) · Split Array Largest Sum (410) · Search a 2D Matrix (74) · Find Peak Element (162) · Time Based Store (981)
Hard	Median of Two Sorted Arrays (4) · Min Max Distance to Gas Station (774)

#Trees → [page](trees.html)

Level	Problems
Easy	Max Depth (104) · Invert Tree (226) · Same Tree (100) · Symmetric Tree (101) · Diameter (543)
Medium	Level Order (102) · Validate BST (98) · LCA (236) · LCA of BST (235) · Kth Smallest in BST (230) · Construct from Pre+In (105) · Right Side View (199) · Path Sum II (113)
Hard	Max Path Sum (124) · Serialise/Deserialise (297)

#Graphs → [page](graphs.html)

Level	Problems
Easy	Number of Islands (200) · Flood Fill (733) · Max Area of Island (695)
Medium	Rotting Oranges (994) · Course Schedule (207) · Course Schedule II (210) · Clone Graph (133) · Pacific Atlantic (417) · Number of Provinces (547) · Surrounded Regions (130) · Word Ladder (127) · Redundant Connection (684) · Network Delay Time (743)
Hard	Alien Dictionary (269) · Word Ladder II (126) · Swim in Rising Water (778)

#Heap → [page](heap.html)

Level	Problems
Easy	Kth Largest in Stream (703) · Last Stone Weight (1046)
Medium	Top K Frequent (347) · Kth Largest in Array (215) · K Closest Points (973) · Task Scheduler (621) · Reorganise String (767) · Meeting Rooms II (253) · Design Twitter (355)
Hard	Find Median from Stream (295) · Merge k Sorted Lists (23) · Smallest Range (632)

#Intervals → [page](intervals.html)

Level	Problems
Easy	Meeting Rooms (252) · Summary Ranges (228)
Medium	Merge Intervals (56) · Insert Interval (57) · Non-overlapping (435) · Meeting Rooms II (253) · Minimum Arrows (452) · Interval Intersections (986) · Car Pooling (1094)
Hard	Employee Free Time (759)

#Dynamic programming → [page](dynamic-programming.html)

Level	Problems
Easy	Climbing Stairs (70) · Min Cost Climbing Stairs (746) · House Robber (198) · House Robber II (213)
Medium	Coin Change (322) · Coin Change II (518) · LIS (300) · LCS (1143) · Word Break (139) · Unique Paths (62) · Partition Equal Subset (416) · Max Product Subarray (152) · Decode Ways (91)
Hard	Edit Distance (72) · Burst Balloons (312) · Regex Matching (10)

#Suggested order across patterns

Do not work down one pattern to completion before starting the next. Interleave, because interleaving is what builds the recognition that the interview tests.

   week 1   hashing easy+medium        two pointers easy
   week 2   two pointers medium        sliding window easy+medium
   week 3   binary search all          REVIEW week 1
   week 4   trees easy+medium          stack easy+medium
   week 5   graphs easy+medium         heap easy+medium
   week 6   REVIEW everything          intervals
   week 7   DP easy+medium             hard problems from earlier patterns
   week 8   REVIEW + mocks

Two review weeks in eight is not generous, it is the minimum. Skipping them to cover more patterns is how people reach week eight having forgotten week one.

#When you are ready

Not "when the list is finished". These:

The 21 starred problems, re-derivable cold in under 25 minutes each
Ten unseen problems, pattern named correctly for eight, in under 60s each
Templates for binary search, BFS, DFS and the monotonic stack typed from memory
At least three mock interviews completed, out loud, with a human