0. Pinned Summary

My time machine, please go back to 2006-09-01 damn hot Hangzhou.

1. Favourate
  • [3] Longest Substring Without Repeating Characters
  • [5] Longest Palindromic Substring
  • [18] 4Sum => [follow up] extend to k-sum
  • [21]/[23] Merge Two Sorted Lists => Merge k Sorted Lists (* big-O analysis)
    • [56] Merge Intervals
    • [88] Merge Sorted Array
    • [4] Median of Two Sorted Arrays
  • [22] Generate Parentheses
  • [24]/[25] Swap Nodes in Pairs => 25. Reverse Nodes in k Group
    • [206] Reverse Linked List (bfs + dfs)
    • [92] Reverse Linked List II
  • [31] Next Permutation
  • [33]/[81] Search in Rotated Sorted Array I/II (weird…but worth noted)
  • [34] - Find First and Last Position of Element in Sorted Array
  • [39]/[40]/[216]/[377] Combination Sum I/II/III/IV (DP solution, not typical dp[]) -> 377’s [Followup]
    • [77] combinations (dfs+bfs)
  • [42] - Trapping Rain Water (多重解法)
  • [44] - Wildcard Matching
  • [48] - Rotate Image
  • [49] Group Anagrams
  • [53] - Maximum Subarray O(N)
  • [55]/[45] Jump Game I/II
  • [62]/[63]/[980] Unique Paths I/II/III ==> extend: III return the actual path instead of path-count
  • [66] - Plus One (clean code)
  • [68] - Text Justification (主要看modular的能力，否则代码太乱)
  • [72] - Edit Distance
    • [161] - One Edit Distance
  • [74]/[240] Search a 2D Matrix I/II (74 binary search 解法)
  • [76] Minimum Window Substring
  • [78]/[90] Subsets I/II
    • [416] Partition Equal Subset Sum
    • [698] Partition to K Equal Sum Subsets
  • [79]/[208]/[212]/[211] Word Search I => Implement Trie Prefix Tree => Word Search II => Add and Search Word (Data structure design)
  • [84] Largest Rectangle in Histogram (hard…if time allows)
    • [85] Maximal Rectangle 最大矩形
  • [94] Binary Tree Inorder Traversal (iteration)
    • [100] Same Tree (二叉树的四种遍历(层序，先序，中序，后序)均有各自的迭代和递归的写法)
  • [96]/[95] Unique Binary Search Trees I/II (II 另类DP)
    • [241] Different Ways to Add Parentheses
  • [97] Interleaving String
  • [103]/[281] Binary Tree Zigzag Level Order => Zigzag Iterator (good [follow-up]) (extend to multi-thread design)
    • [6] ZigZag <= bad one, skip it
  • [105] Construct Binary Tree from Preorder and Inorder Traversal
    • [106] Construct Binary Tree from Inorder and Postorder Traversal
    • [889] Construct Binary Tree from Preorder and Postorder Traversal
    • [536] Construct Binary Tree from String (start from no ‘-‘, then expand to support ‘-‘)
    • [108] Convert Sorted Array to Binary Search Tree
    • [109] Convert Sorted List to Binary Search Tree
    • [538] Convert BST to Greater Tree (easy to warm up)
    • [114] - Flatten Binary Tree to Linked List
    • [116] - Populating Next Right Pointers in Each Node (bfs + dfs)
  • [111] Minimum Depth of Binary Tree (bfs + dfs)
  • [116]/[117] Populating Next Right Pointers in Each Node I/II (bfs + dfs)
    • [199] Binary Tree Right Side View (面试简单热身题)
    • [314] Binary Tree Vertical Order Traversal 二叉树的竖直遍历
  • [121]/[122]/[123]/[188] Best Time to Buy and Sell Stock I,II,III,IV
    • [309] Best Time to Buy and Sell Stock with Cooldown
    • [714] Best Time to Buy and Sell Stock with Transaction Fee
  • [127]/[126] Word Ladder I/II
  • [131]/[132] Palindrome Partitioning I/II => List作为dp，不是数组[]作为dp
  • [136]/[137]/[260] Single Number I,II,III
  • [138] Copy List with Random Pointer => no extra space solution
    • skip-[382] Linked List Random Node => math-stats “rn = rd.nextInt(idx);” 水塘抽样 Reservoir Sampling
  • [141]/[142] Linked List Cycle I/II
  • [146]/[460] LRU Cache / LFU Cache
  • [148] Sort List (top down, bottom up => solution analysis)
  • [155] Min Stack (easy, but good test for data structure)
  • [159]/[340] Longest Substring with At Most [[Two K]]() [Distinct] Characters
  • [151]/[186]/[557] Reverse Words in a String I (extra space -> in place) => II/III
    • [344]/[541]/[345] 344 Reverse String => 541 Reverse String II => 345 Reverse Vowels of a String
  • [164] - [Hard] Maximum Gap
  • [187] - Repeated DNA Sequences (expand to space saving via mask encoding, data compression)
  • [189] Rotate Array
  • [192]/[347]/[692] Word Frequency => Top K Frequent Elements => Top K Frequent Words
    • [193] Valid Phone Numbers (shell script)
  • [198]/[213]/[337] House Robber I, II, III
  • [200]/[305] Number of Islands -> extend from 2D to 3D
  • [215] Kth Largest Element in an Array
  • [217]/[219]/[220] Contains Duplicate I/II/III
  • [218][Hard] The Skyline Problem
  • [222] Count Complete Tree Nodes
  • [224]/[227]/[772] [Tricky-NotSoGood] Basic Calculator I/II/III
  • [225] Implement Stack using Queues
    • [232] Implement Queue using Stacks
  • [230] - Kth Smallest Element in a BST (multiple solutions, + good [follow up])
  • [235]/[236] Lowest Common Ancestor of a Binary Search Tree (bfs+dfs) => Lowest Common Ancestor of a Binary Tree
  • [238] Product of Array Except Self => [follow up]
  • [239] Sliding Window Maximum
  • [241] Different Ways to Add Parentheses
  • [243]/[244]/[245] Shortest Word Distance I,II,III
  • [249] Group Shifted Strings
  • [250] Count Univalue Subtrees (bottom up and top down)
  • [251] Flatten 2D Vector => great hints
  • [252]/[253]/[1229] Meeting Rooms I,II => Meeting Scheduler
  • [254] Factor Combinations
  • [255] Verify Preorder Sequence in Binary Search Tree (bfs + dfs)
    • [98] Validate Binary Search Tree (bfs + dfs)
    • [173] Binary Search Tree Iterator (multiple data structure)
    • [1586] Binary Search Tree Iterator II
  • [266]/[267] Palindrome Permutation I/II
  • [269] [Hard] Alien Dictionary (dfs+bfs)
  • [270]/[272] Closest Binary Search Tree Value I,II => optimize solution, from binary-tree to BST
  • [273] Integer to English Words (real use case)
  • [277] Find the Celebrity ([follow up] reduce api call)
  • [280]/[324] Wiggle Sort I,II => o(N) solution (I里，需要数学证明，直接和前一个swap是可行)(II也一般)
    • [376] Wiggle Subsequence (optimize: dp[] to single variables)
  • [282] Expression Add Operators
  • [290]/[291] Word Pattern I,II
  • [293]/[294] Flip Game I,II
  • [296] Best Meeting Point
  • [297] Serialize and Deserialize [Binary Tree]
  • [302] Smallest Rectangle Enclosing Black Pixels
  • [303] Range Sum Query - Immutable (这一组是各种不常见的变种树结构，有精力再看这组题)
    • [304] Range Sum Query 2D - Immutable
    • [307] Range Sum Query - Mutable
    • [308] Range Sum Query 2D - Mutable
  • [310] Minimum Height Trees
  • [318] Maximum Product of Word Lengths
  • [322] Coin Change
  • [323] - Number of Connected Components in an Undirected Graph (connected component search)
  • [333] Largest BST Subtree => o(N) (dfs+bfs)
  • [334] Increasing Triplet Subsequence => optimize: O(n) time complexity and O(1) space complexity
  • [339]/[364] Nested List Weight Sum I,II
  • [341] Flatten Nested List Iterator (dfs+bfs)
  • [346] Moving Average from Data Stream => can extend to a system design question
  • [349]/[350] Intersection of Two Arrays I/II (easy to complex)
  • [354] Russian Doll Envelopes => 1D=>2D [follow-up]: can rotate
    • [1691] Maximum Height by Stacking Cuboids => 2D to 3D
    • [300] Longest Increasing Subsequence => 1D [follow up]
  • [358] Rearrange String k Distance Apart
  • [359] Logger Rate Limiter (extend to throttle, load balancing, etc.)
    • [362] Design Hit Counter (counter too much, then need rate limiter) ([follow up])
  • [360] Sort Transformed Array => optimize to O(N)
  • [368] Largest Divisible Subset
  • [370] Range Addition (累加的累加, 一维的array)
    • [598] Range Addition II 二维的matrix
  • [371] Sum of Two Integers
  • [373] Find K Pairs with Smallest Sums => start from 2 to K
  • [380] Insert Delete GetRandom O(1)
    • [381] Insert Delete GetRandom O(1) - Duplicates allowed
  • [392] Is Subsequence => [follow-up]: large streaming input => can extend to a design question
  • [412]/[1195] Fizz Buzz => concurrency Multithreaded
  • [485]/[487]/[1004] Max Consecutive Ones I/II/III (good [followup], flip k times instead 1 time, input is an infinite stream)
  • [516]/[1682] Longest Palindromic Subsequence I/II
    • [298] Binary Tree Longest Consecutive Sequence
  • [539] Minimum Time Difference (real usecase)
  • [560] Subarray Sum Equals K => 持续优化
  • [650]/[651] 2 Keys Keyboard / 4 Keys Keyboard (dp/recursion convert)
  • [653] Two Sum IV - Input is a BST (bfs+dfs)
  • [811] Subdomain Visit Count
    • [718] Maximum Length of Repeated Subarray (extend to: longest shared view history of 2 users)
  • [1059] All Paths from Source Lead to Destination (typical graph traverse, dfs+bfs)
  • [1114] Print in Order (latch)
  • [1135] Connecting Cities With Minimum Cost (UnionFind, 两种graph处理方式)
    • [1976] Number of Ways to Arrive at Destination
  • [1236]/[1242] Web Crawler / Multithreaded (with [follow up]) => start from single thread, then multi-thread, then multi-host
  • [1258] Synonymous Sentences (big-O analysis)
  • [1265] Print Immutable Linked List in Reverse (good [follow up])
  • [1419] Minimum Number of Frogs Croaking (increasing then converge)
  • [1597] Build Binary Expression Tree From Infix Expression (in-order style)
    • [1628] Design an Expression Tree With Evaluate Function (post-order, with [follow up])
  • [1612] Check If Two Expression Trees are Equivalent ([follow-up] extend from + only to +/-/*)
  • [1673] Find the Most Competitive Subsequence (单调栈)
  • [1746] Maximum Subarray Sum After One Operation (good dp)
  • [1788] Maximize the Beauty of the Garden (brute force to optimize, nice way to check 2nd appearance)
2. tag [SQL]
  • [182]/[196] Duplicate Emails => Delete Duplicate Emails (delete clause)
    • insert clause: INSERT INTO table_name (column1, column2, column3, …) VALUES (value1, value2, value3, …)
    • update clause: update table set a=1 where b=2
  • [181]/[184]/[185] 181. Employees Earning More Than Their Managers => 184. Department Highest Salary => [185] Department Top Three Salaries
    • [176]/[177] Second Highest Salary => Nth Highest Salary
  • [262] Trips and Users (xxxDate between a and b)
  • [550] - Game Play Analysis IV (DATEDIFF)
  • [612] Shortest Distance in a Plane (cross join)
  • [1076] Project Employees II (max record’s rest columns)
  • [1084] Sales Analysis III (date compare)
  • [1270] All People Report to the Given Manager (sql recursion)
  • [1407] Top Travellers (ifnull)
  • [1479] Sales by Day of the Week (dayofweek)
  • [1555]/[1587] Bank Account Summary I/II
  • [1596] The Most Frequently Ordered Products for Each Customer (partition over)
  • [1613] common table expression (连续id)
  • [1661] Average Time of Process per Machine
  • [1699] Number of Calls Between Two Persons (WITH xxx AS)
3. tag [top-100]
4. tag [design]
  • [170] Two Sum III - Data structure design
  • [211] Design Add and Search Words Data Structure
  • [348] Design Tic-Tac-Toe
  • [353] Design Snake Game (real life example)
  • [355] Design Twitter
  • [362] Design Hit Counter => [follow-up] question
  • [379] Design Phone Directory
  • [588] Design In-Memory File System
  • [604] Design Compressed String Iterator
    • [443] String Compression
  • [622] Design Circular Queue
    • [641] Design Circular Deque
  • [631] Design Excel Sum Formula
  • [635] Design Log Storage System (asked by all interviews for data positions, eg. Apple)
    • link to my own resume, cloudwatch team
  • [642] Design Search Autocomplete System (asked by Zenefits interview) (TrieNode for search)
  • [705] Design HashSet
  • [706] Design HashMap
  • [707] Design Linked List
  • [1188] Design Bounded Blocking Queue (multi-thread)
  • [1206] [Hard] Design Skiplist
  • [1244] Design A Leaderboard (binary search)
  • [1381] Design a Stack With Increment Operation
  • [1396] Design Underground System
  • [1472] Design Browser History
  • [1500] Design a File Sharing System ([follow up])
  • [1603] Design Parking System
  • [1628] Design an Expression Tree With Evaluate Function
  • [1656] Design an Ordered Stream 没什么意思的题，可以跳过
  • [1670] Design Front Middle Back Queue
  • [1797] Design Authentication Manager

Revisit

  • 15
  • 1674
  • 1539
  • 1653 枚举法
  • 1636
  • 1554
  • 1064
  • 1380
  • 1120

Bad - skip

  • 164 Maximum Gap
  • 233 Number of Digit One
    • pure math and pattern
  • 302 Smallest Rectangle Enclosing Black Pixels
  • 311 Sparse Matrix Multiplication -> math
  • 319 Bulb Switcher (math and pattern finding)
  • 327 Count of Range Sum
  • 357 Count Numbers with Unique Digits (math and pattern finding)
  • 365 Water and Jug Problem (math)
  • 372 Super Pow (math)
  • 375 (pending) Guess Number Higher or Lower II (Minimax算法)
  • 390 Elimination Game
  • 391 Perfect Rectangle
  • 393 UTF-8 Validation
  • 672 Bulb Switcher II

All Problems

All Solutions