Records of Review type problems date understanding DFS Combinations Of Coins May 17, 2022 moderate DFS All Permutations I May 17, 2022 weak DFS All Subsets I May 17, 2022 moderate DFS All Valid Permutations Of Parentheses I May 17, 2022 moderate Recursion Maximum Path Sum Binary Tree II May 18, 2022 weak Recursion Spiral Order Traverse I May 18, 2022 weak Recursion N Queens May 18, 2022 weak Recursion Reverse Linked List In Pairs May 18, 2022 weak Recursion String Abbreviation Matching May 18, 2022 very weak Recursion Store Number Of Nodes In Left Subtree May 19, 2022 moderate Recursion Lowest Common Ancestor I May 19, 2022 moderate Recursion Spiral Order Traverse II May 19, 2022 weak Recursion Reverse Binary Tree Upside Down May 19, 2022 weak DFS All Permutations I May 21, 2022 moderate Recursion String Abbreviation Matching May 21, 2022 moderate Recursion N Queens May 21, 2022 moderate Recursion Spiral Order Traverse II May 21, 2022 weak Recursion N Queens May 30, 2022 moderate Recursion String Abbreviation Matching May 30, 2022 moderate DFS All Permutations I May 30, 2022 strong Heap K Smallest In Unsorted Array May 31, 2022 weak BFS Get Keys In Binary Tree Layer By Layer May 31, 2022 weak BFS Bipartite May 31, 2022 weak BFS Check If Binary Tree Is Completed May 31, 2022 moderate BFS Kth Smallest Number In Sorted Matrix May 31, 2022 weak DFS All Valid Permutations Of Parentheses I Jun 1, 2022 strong DFS Combinations Of Coins Jun 1, 2022 strong Recursion Spiral Order Traverse II Jun 1, 2022 moderate Recursion Maximum Path Sum Binary Tree I Jun 1, 2022 weak Recursion Maximum Path Sum Binary Tree II Jun 1, 2022 moderate Recursion Max Path Sum From Leaf To Root Jun 1, 2022 weak Recursion Binary Tree Path Sum To Target III Jun 1, 2022 weak Recursion Reverse Linked List In Pairs Jun 2, 2022 moderate Recursion Maximum Path Sum Binary Tree III Jun 2, 2022 moderate Recursion Flatten Binary Tree to Linked List Jun 3, 2022 weak DFS All Valid Permutations Of Parentheses I Jun 9, 2022 strong DFS Combinations Of Coins Jun 9, 2022 moderate Recursion Spiral Order Traverse II Jun 9, 2022 weak DFS All Subsets I Jun 10, 2022 strong Recursion Maximum Path Sum Binary Tree I Jun 10, 2022 moderate Recursion Maximum Path Sum Binary Tree II Jun 10, 2022 moderate Recursion Maximum Path Sum Binary Tree III Jun 10, 2022 moderate Recursion Binary Tree Path Sum To Target III Jun 10, 2022 weak Heap K Smallest In Unsorted Array Jun 11, 2022 moderate BFS Get Keys In Binary Tree Layer By Layer Jun 11, 2022 moderate BFS Bipartite Jun 11, 2022 weak BFS Seven Puzzle Jun 11, 2022 weak BFS Word Ladder II Jun 13, 2022 weak BFS Word Ladder Jun 13, 2022 weak BFS Largest Product Of Length Jun 13, 2022 weak BFS Kth Smallest With Only 3, 5, 7 As Factors Jun 13, 2022 weak BFS Kth Closest Point To <0,0,0> Jun 13, 2022 weak BFS Kth Closest Point To <0,0,0> Jun 14, 2022 moderate BFS Seven Puzzle Jun 14, 2022 moderate Recursion N Queens Jun 14, 2022 strong Recursion String Abbreviation Matching Jun, 2022 moderate BFS Course Schedule II Jun 16, 2022 weak BFS Place To Put The Chair I Jun 16, 2022 weak BFS Alien Dictionary Jun 16, 2022 weak DFS Two Subsets With Min Difference Jun 17, 2022 weak DFS All Subsets II of Size K Jun 18, 2022 weak DFS All Subsets of Size K Jun 18, 2022 moderate DFS All Subsets II Jun 18, 2022 moderate DFS All Valid Permutations Of Parentheses II Jun 18, 2022 weak DFS All Valid Permutations Of Parentheses III Jun 18, 2022 weak DFS Factor Combinations Jun 18, 2022 weak DFS All Permutations of Subsets Jun 19, 2022 moderate DFS Keep Distance For Identical Elements Jun 19, 2022 weak DFS Combinations For Telephone Pad I Jun 19, 2022 weak DFS Restore IP Addresses Jun 19, 2022 weak Recursion Reconstruct Binary Tree With Preorder And Inorder Jun 21, 2022 weak Recursion Reconstruct Binary Search Tree With Postorder Traversal Jun 21, 2022 weak Recursion Reconstruct Binary Tree With Levelorder And Inorder Jun 21, 2022 weak BFS Seven Puzzle Jun 22, 2022 moderate Recursion String Abbreviation Matching Jun 22, 2022 moderate DFS Two Subsets With Min Difference Jun 22, 2022 moderate DFS All Subsets II of Size K Jun 22, 2022 moderate DFS All Valid Permutations Of Parentheses III Jun 22, 2022 moderate Dynamic Programming Longest Ascending SubArray Jun 24, 2022 moderate Dynamic Programming Max Product Of Cutting Rope Jun 24, 2022 weak Dynamic Programming Dictionary Word I Jun 24, 2022 weak Two Pointers Find the Duplicate Number Jun 27, 2022 moderate Dynamic Programming Array Hopper I Jun 28, 2022 moderate Dynamic Programming Array Hopper II Jun 28, 2022 moderate Dynamic Programming Largest Subarray Sum Jun 28, 2022 moderate Dynamic Programming Dictionary Word I Jun 29, 2022 moderate Dynamic Programming Edit Distance Jun 29, 2022 weak Dynamic Programming Largest Square Of 1s Jul 1, 2022 weak Dynamic Programming Longest Consecutive 1s Jul 1, 2022 strong Dynamic Programming Longest Cross Of 1s Jul 1, 2022 moderate Dynamic Programming Largest X Of 1s Jul 1, 2022 weak Dynamic Programming Largest Square Surrounded By One Jul 1, 2022 weak Dynamic Programming Largest Square Of Matches Jul 2, 2022 weak Dynamic Programming Longest Ascending Subsequence Jul 2, 2022 weak Dynamic Programming Longest Ascending Subsequence II Jul 3, 2022 weak Dynamic Programming Count Ascending Subsequence Jul 3, 2022 moderate Dynamic Programming Largest Set Of Points With Positive Slope Jul 3, 2022 weak Dynamic Programming Longest Common Substring Jul 3, 2022 weak Dynamic Programming Longest Common Subsequence Jul 3, 2022 weak HashTable Top K Frequent Words Jul 4, 2022 moderate HashTable Missing Number I Jul 4, 2022 moderate HashTable Common Numbers Of Two Sorted Arrays(Array version) Jul 4, 2022 moderate HashTable Remove Certain Characters Jul 4, 2022 moderate String Remove Spaces Jul 4, 2022 weak String Remove Adjacent Repeated Characters I Jul 4, 2022 moderate String Remove Adjacent Repeated Characters IV Jul 4, 2022 moderate String Reverse String Jul 6, 2022 strong String Reverse Words In A Sentence I Jul 6, 2022 moderate String Right Shift By N Characters Jul 6, 2022 moderate String String Replace Jul 6, 2022 weak String All Permutations II Jul 8, 2022 moderate String Reoder Array Jul 8, 2022 weak String Compress String II Jul 8, 2022 weak String Decompress String II Jul 8, 2022 weak String Decompress String II Jul 9, 2022 weak String Longest Substring Without Repeating Characters Jul 9, 2022 weak String All Anagrams Jul 9, 2022 weak String Longest subarray contains only 1s Jul 9, 2022 weak Recursion Store Number Of Nodes In Left Subtree Jul 10, 2022 moderate Recursion Lowest Common Ancestor I Jul 10, 2022 moderate BFS Check If Binary Tree Is Completed Jul 10, 2022 moderate Recursion Reverse Linked List In Pairs Jul 10, 2022 moderate Recursion Max Path Sum From Leaf To Root Jun 10, 2022 moderate String Longest subarray contains only 1s Jul 11, 2022 moderate String Longest Repeating Character Replacement (leetcode 424) Jul 11, 2022 moderate Binary Tree Lowest Common Ancestor II Jul 12, 2022 moderate Binary Tree Lowest Common Ancestor Binary Search Tree I Jul 12, 2022 moderate Binary Tree Lowest Common Ancestor V Jul 12, 2022 moderate Binary Tree Lowest Common Ancestor VI Jul 12, 2022 moderate Binary Tree Lowest Common Ancestor IV Jul 12, 2022 moderate Array Array Deduplication I Jul 12, 2022 moderate Array Array Deduplication II Jul 12, 2022 moderate Array Array Deduplication III Jul 12, 2022 moderate Array Array Deduplication IV Jul 14, 2022 moderate Array Largest ans Smallest Jul 14, 2022 moderate Array Largest ans Second Largest Jul 14, 2022 moderate Array Closest Number In Binary Search Tree II Jul 14, 2022 weak Array Move 0s To The End II Jul 15, 2022 moderate Array Rotate Matrix Jul 15, 2022 moderate Array Get Keys In Binary Tree Layer By Layer Zig-Zag Order Jul 15, 2022 weak Binary Tree Search In Binary Search Tree Jul 15, 2022 moderate Binary Tree Delete In Binary Search Tree Jul 15, 2022 weak Sorting Merge Sort Jul 16, 2022 weak Sorting Quick Sort Jul 16, 2022 weak Sorting Rainbow Sort Jul 16, 2022 weak Array 2 Sum Jul 17, 2022 weak Array 2 Sum All Pair I Jul 17, 2022 weak Array 2 Sum All Pair II Jul 18, 2022 weak Array 3 Sum Jul 18, 2022 weak Array 4 Sum Jul 18, 2022 weak Binary Tree Closest Number In Binary Search Tree Jul 18, 2022 weak Binary Tree Largest Number Smaller In Binary Search Tree Jul 18, 2022 weak Linked List Deep Copy Linked List With Random Pointer Jul 18, 2022 weak DFS Deep Copy Undirected Graph Jul 18, 2022 weak Array Common Numbers Of Two Arrays I(Array version) Jul 18, 2022 moderate Array Merge K Sorted Array Jul 23, 2022 weak Linked List Merge K Sorted List Jul 23, 2022 weak Array Common Elements In Three Sorted Array Jul 23, 2022 weak Array Common Elements In K Sorted Array Jul 23, 2022 weak Array Max Water Trapped I Jul 23, 2022 weak BFS Kth Smallest Sum In Two Sorted Array Jul 24, 2022 weak Array Maximum Values Of Size K Sliding Windows Jul 24, 2022 weak Array Majority Number I Jul 24, 2022 weak Array Majority Number II Jul 24, 2022 weak OOD First Non-Repeating Character In Stream Jul 26, 2022 weak OOD Implement LRU Cache Jul 26, 2022 weak Array Kth Smallest In Two Sorted Arrays Jul 26, 2022 weak Array Majority Number III Jul 26, 2022 weak Linked List Check If Linked List Is Palindrome Jul 29, 2022 weak Linked List Remove Linked List Element Jul 29, 2022 weak Binary Search Search In Unknown Sized Sorted Array Jul 29, 2022 weak Stack & Queue Deque By Three Stacks Jul 29, 2022 weak Binary Tree Pre-order Traversal Of Binary Tree (Iterative) Jul 29, 2022 weak Binary Tree In-order Traversal Of Binary Tree (Iterative) Jul 29, 2022 weak Dynamic Programming Minimum Cuts For Palindrome Jul 30, 2022 weak Dynamic Programming Can I Win II Jul 30, 2022 weak Linked List Reorder Linked List Jul 30, 2022 weak DFS All Subsets II Aug 26, 2022 moderate Recursion Reconstruct Binary Tree With Levelorder And Inorder Aug 26, 2022 moderate Dynamic Programming Coin Change Aug 31, 2022 weak Dynamic Programming Coin Change 2 Aug 31, 2022 weak Dynamic Programming Target Sum Aug 31, 2022 weak OOD Parking Lot Google Doc Link

Bit Representation Using bit operation won’t change time complexity. positive number: the most significant bit is sign bit. 0: positive number; 1: negative number An integer is 32 bit. java (max)int = 2^31 - 1 = 2147483647; java (max)long = 2^63 - 1

Theory Key points for using linked list: when we are trying to dereference a ListNode, we should check if the node is null, or we may get an NPE (Null Pointer Exception).

Recursion function calls itself. Boil down a big problem to smaller ones. Implementation: Base case: smallest problem to solve. Subproblem: smaller size of the pre-problem. (when we have the result of the subproblem, the what you need to do to solve the pre-problem based on subproblem) Recursive rule: how to make the problem smaller.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 // Java code to illustrate StringBuilder import java.

Theory Threads can be created by using two mechanisms: Extending the Thread class Implementing the Runnable Interface Thread creation by extending the Thread class We create a class that extends the java.