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Problems frequently reported at Google, in course order. Solve the must-dos first.

65 easy142 medium45 hardSimulation ×12 Hash lookup (seen before?) ×12 Frequency counting ×11 Bit tricks (shift, mask, lowest set bit) ×9 Union-Find (DSU) ×9 1D DP ×8

02 · Arrays & Strings in Java

06 · Mindset & Problem Solving

#1480Running Sum of 1d ArrayEasy
#412Fizz BuzzEasy

07 · Big-O & Complexity

#69Sqrt(x)Easy

08 · Recursion

#509Fibonacci NumberEasymust-do
#344Reverse StringEasy
#24Swap Nodes in PairsMedium
#50Pow(x, n)Mediummust-do
#779K-th Symbol in GrammarMedium

09 · Math & Number Theory

#66Plus OneEasy
#9Palindrome NumberEasy
#202Happy NumberEasy
#7Reverse IntegerMediummust-do
#43Multiply StringsMediummust-do
#528Random Pick with WeightMediummust-do
#149Max Points on a LineHard

10 · Arrays & Strings

#122Best Time to Buy and Sell Stock IIMedium
#918Maximum Sum Circular SubarrayMedium
#169Majority ElementEasymust-do
#14Longest Common PrefixEasy
#189Rotate ArrayMedium
#41First Missing PositiveHardmust-do

11 · Matrices & Simulation

#48Rotate ImageMediummust-do
#6Zigzag ConversionMedium
#289Game of LifeMediummust-do

12 · Two Pointers

#977Squares of a Sorted ArrayEasy
#167Two Sum II - Input Array Is SortedMediummust-do
#163Sum ClosestMedium
#27Remove ElementEasy
#26Remove Duplicates from Sorted ArrayEasy
#88Merge Sorted ArrayEasymust-do
#392Is SubsequenceEasy

13 · Sliding Window

#643Maximum Average Subarray IEasy
#424Longest Repeating Character ReplacementMediummust-do
#567Permutation in StringMediummust-do
#438Find All Anagrams in a StringMediummust-do
#209Minimum Size Subarray SumMediummust-do
#1004Max Consecutive Ones IIIMediummust-do
#219Contains Duplicate IIEasy
#30Substring with Concatenation of All WordsHard

14 · Hashing & Prefix Sums

#1Two SumEasymust-do
#217Contains DuplicateEasymust-do
#409Longest PalindromeEasy
#205Isomorphic StringsEasy
#290Word PatternEasy
#128Longest Consecutive SequenceMediummust-do
#303Range Sum Query - ImmutableEasymust-do
#304Range Sum Query 2D - ImmutableMedium
#525Contiguous ArrayMedium

15 · Linked Lists

#206Reverse Linked ListEasymust-do
#21Merge Two Sorted ListsEasymust-do
#141Linked List CycleEasymust-do
#19Remove Nth Node From End of ListMediummust-do
#142Linked List Cycle IIMediummust-do
#287Find the Duplicate NumberMediummust-do
#2Add Two NumbersMediummust-do
#25Reverse Nodes in k-GroupHardmust-do

16 · Stacks & Monotonic Stack

#155Min StackMediummust-do
#150Evaluate Reverse Polish NotationMediummust-do
#739Daily TemperaturesMediummust-do
#496Next Greater Element IEasy
#84Largest Rectangle in HistogramHardmust-do
#853Car FleetMedium
#394Decode StringMediummust-do
#71Simplify PathMedium
#32Longest Valid ParenthesesHard
#844Backspace String CompareEasy

17 · Queues & Deques

#933Number of Recent CallsEasy
#362Design Hit CounterMedium
#239Sliding Window MaximumHardmust-do
#950Reveal Cards In Increasing OrderMedium
#2073Time Needed to Buy TicketsEasy
#862Shortest Subarray with Sum at Least KHard

18 · Binary Search

#704Binary SearchEasymust-do
#35Search Insert PositionEasymust-do
#74Search a 2D MatrixMedium
#153Find Minimum in Rotated Sorted ArrayMediummust-do
#875Koko Eating BananasMediummust-do
#1011Capacity To Ship Packages Within D DaysMedium
#162Find Peak ElementMediummust-do
#4Median of Two Sorted ArraysHard
#278First Bad VersionEasymust-do
#658Find K Closest ElementsMedium

19 · Sorting

#912Sort an ArrayMediummust-do
#179Largest NumberMediummust-do
#274H-IndexMedium
#493Reverse PairsHard
#324Wiggle Sort IIMedium

20 · String Parsing & Expression Evaluation

#13Roman to IntegerEasymust-do
#151Reverse Words in a StringMediummust-do
#8String to Integer (atoi)Mediummust-do
#224Basic CalculatorHardmust-do
#271Encode and Decode StringsMediummust-do
#68Text JustificationHard

21 · Tree Traversals

#94Binary Tree Inorder TraversalEasymust-do
#637Average of Levels in Binary TreeEasy
#103Binary Tree Zigzag Level Order TraversalMedium
#117Populating Next Right Pointers in Each Node IIMedium
#662Maximum Width of Binary TreeMedium
#863All Nodes Distance K in Binary TreeMediummust-do

22 · Tree Recursion

#110Balanced Binary TreeEasy
#100Same TreeEasy
#226Invert Binary TreeEasymust-do
#101Symmetric TreeEasymust-do
#572Subtree of Another TreeEasymust-do
#129Sum Root to Leaf NumbersMedium
#1448Count Good Nodes in Binary TreeMediummust-do
#222Count Complete Tree NodesMedium
#437Path Sum IIIMediummust-do
#543Diameter of Binary TreeEasymust-do
#105Construct Binary Tree from Preorder and Inorder TraversalMedium
#106Construct Binary Tree from Inorder and Postorder TraversalMedium
#114Flatten Binary Tree to Linked ListMedium

23 · Binary Search Trees

#700Search in a Binary Search TreeEasy
#701Insert into a Binary Search TreeMedium
#230Kth Smallest Element in a BSTMediummust-do
#235Lowest Common Ancestor of a Binary Search TreeMediummust-do
#285Inorder Successor in BSTMediummust-do
#530Minimum Absolute Difference in BSTEasy
#108Convert Sorted Array to Binary Search TreeEasy
#173Binary Search Tree IteratorMediummust-do
#220Contains Duplicate IIIHard
#729My Calendar IMedium

24 · Heaps & Priority Queues

#703Kth Largest Element in a StreamEasymust-do
#1046Last Stone WeightEasy
#973K Closest Points to OriginMediummust-do
#355Design TwitterMedium
#767Reorganize StringMedium
#373Find K Pairs with Smallest SumsMedium
#502IPOHard
#632Smallest Range Covering Elements from K ListsHard
#692Top K Frequent WordsMedium

25 · Tries

#208Implement Trie (Prefix Tree)Mediummust-do
#211Design Add and Search Words Data StructureMediummust-do
#212Word Search IIHardmust-do
#648Replace WordsMedium
#1268Search Suggestions SystemMediummust-do
#336Palindrome PairsHard

26 · Graphs: BFS & DFS

#733Flood FillEasy
#695Max Area of IslandMediummust-do
#133Clone GraphMediummust-do
#54201 MatrixMediummust-do
#286Walls and GatesMedium
#417Pacific Atlantic Water FlowMedium
#130Surrounded RegionsMedium
#785Is Graph Bipartite?Mediummust-do
#1091Shortest Path in Binary MatrixMediummust-do
#909Snakes and LaddersMedium
#1197Minimum Knight MovesMedium
#433Minimum Genetic MutationMedium

27 · Shortest Paths

#743Network Delay TimeMediummust-do
#787Cheapest Flights Within K StopsMediummust-do
#1514Path with Maximum ProbabilityMedium
#1631Path With Minimum EffortMediummust-do
#778Swim in Rising WaterHard
#1368Minimum Cost to Make at Least One Valid Path in a GridHard
#2290Minimum Obstacle Removal to Reach CornerHardmust-do
#1976Number of Ways to Arrive at DestinationMedium

28 · Union-Find & Minimum Spanning Trees

#547Number of ProvincesMediummust-do
#323Number of Connected Components in an Undirected GraphMedium
#684Redundant ConnectionMediummust-do
#261Graph Valid TreeMediummust-do
#721Accounts MergeMediummust-do
#1584Min Cost to Connect All PointsMediummust-do

29 · Topological Sort

#269Alien DictionaryHardmust-do
#802Find Eventual Safe StatesMedium
#2050Parallel Courses IIIHard
#310Minimum Height TreesMediummust-do
#329Longest Increasing Path in a MatrixHardmust-do
#2392Build a Matrix With ConditionsHard

30 · Advanced Graphs: Euler Paths, Bridges & SCCs

#332Reconstruct ItineraryHardmust-do
#1192Critical Connections in a NetworkHardmust-do
#399Evaluate DivisionMediummust-do
#815Bus RoutesHard

31 · Backtracking

#78SubsetsMediummust-do
#77CombinationsMediummust-do
#90Subsets IIMediummust-do
#46PermutationsMediummust-do
#40Combination Sum IIMedium
#17Letter Combinations of a Phone NumberMedium
#22Generate ParenthesesMediummust-do
#131Palindrome PartitioningMedium
#51N-QueensHardmust-do
#52N-Queens IIHard
#37Sudoku SolverHard

32 · Greedy Algorithms

#55Jump GameMediummust-do
#45Jump Game IIMediummust-do
#134Gas StationMediummust-do
#1899Merge Triplets to Form Target TripletMedium
#763Partition LabelsMedium
#678Valid Parenthesis StringMediummust-do
#846Hand of StraightsMedium
#135CandyHard

33 · Intervals

#228Summary RangesEasy
#252Meeting RoomsEasy
#57Insert IntervalMediummust-do
#435Non-overlapping IntervalsMediummust-do
#452Minimum Number of Arrows to Burst BalloonsMedium
#759Employee Free TimeHard
#1851Minimum Interval to Include Each QueryHard

34 · Bit Manipulation

#136Single NumberEasymust-do
#191Number of 1 BitsEasymust-do
#338Counting BitsEasymust-do
#268Missing NumberEasymust-do
#67Add BinaryEasymust-do
#371Sum of Two IntegersMedium
#137Single Number IIMedium
#260Single Number IIIMedium
#231Power of TwoEasy
#201Bitwise AND of Numbers RangeMedium

35 · Dynamic Programming

#70Climbing StairsEasymust-do
#746Min Cost Climbing StairsEasy
#213House Robber IIMediummust-do
#91Decode WaysMediummust-do
#152Maximum Product SubarrayMediummust-do
#518Coin Change IIMedium
#377Combination Sum IVMedium
#416Partition Equal Subset SumMediummust-do
#62Unique PathsMediummust-do
#63Unique Paths IIMedium
#64Minimum Path SumMedium
#120TriangleMedium
#221Maximal SquareMedium
#1143Longest Common SubsequenceMediummust-do
#72Edit DistanceMediummust-do
#312Burst BalloonsHard

36 · DP Patterns

#309Best Time to Buy and Sell Stock with CooldownMediummust-do
#714Best Time to Buy and Sell Stock with Transaction FeeMedium
#123Best Time to Buy and Sell Stock IIIHard
#188Best Time to Buy and Sell Stock IVHardmust-do
#10Regular Expression MatchingHardmust-do
#97Interleaving StringMediummust-do
#115Distinct SubsequencesHard
#647Palindromic SubstringsMediummust-do
#494Target SumMediummust-do
#1235Maximum Profit in Job SchedulingHardmust-do
#486Predict the WinnerMedium

37 · Design Data Structures

#981Time Based Key-Value StoreMediummust-do
#2013Detect SquaresMedium
#895Maximum Frequency StackHardmust-do
#588Design In-Memory File SystemHard

38 · Segment Trees, Fenwick & String Matching

#307Range Sum Query - MutableMediummust-do
#315Count of Smaller Numbers After SelfHardmust-do
#28Find the Index of the First Occurrence in a StringEasymust-do
#459Repeated Substring PatternEasy
#1392Longest Happy PrefixHardmust-do
#214Shortest PalindromeHard
#1044Longest Duplicate SubstringHard