LeetCode-NoteBook-Python3

Goal

Although the Notebook contains a bunch of problem solutions, we demand more. The notebook aims at helping us become skilled at solving problems using Data structures, Algorithms and tricks properly. Most problems will be hard with hands dirty if one uses "Brute Force", but if we work on them with proper Data structures, Algorithms and tricks, the life will become quite easy as well as with a high quality yield. Instead of through hard working on problems again and again, our motivation is to develop problem-solving skills wisely by applying and deveoping meta-skills such as abstracting, grouping, finding properties, connecting, comparing, evaluating, etc.. (Sounds like working with Category Theory, doesn't it?) Parallelly, there will be a collection of common procedures and steps, guidelines and tips available.

Furthermore, through this project, we hope to collect and develop (usually by abstracting) strategy for developing meta-skills useful in developing problem-solving skills. They can be viewed layer by layer vertically. And horizontally, we need to supply common procedures and steps, guidelines and tips by abstracting. These abilities and the strategy are common in other domains of materials. The higher demand will motivate us through the journey.

How to use LeetCode

Therefore, LeetCode problems are no more isolated individual problems, but a repository of problems with hidden connections of similarity and variance. We need to manage time wisely instead of hacking on everyone starting from problem #1 which is frustrating. Note LeetCode is NOT the place for learning fundamental concepts, we should do that before starting LeetCode. The repository are trainings for developing individual skills and also for applying them flexibly and logically. Moreover, the problems are putting together for abstracting the common procedures and steps for solving problems.

Although it will be slower at the beginning, but it will become much faster at the end with additional gain of higher level abilities.

Map

The knowledges are organized in groups of "Analysis-Solutions", "Algorithms" and "Data-structures". More than a structure, with time passing, we should have a virtual map in our brain for contents as nodes and connections as pathes. That is, LeetCode problems are no more in a linearly ordered sequence, but a graph of nodes and connections.

Schedule

• One month on "Easy" questions: They provide us the chance for understanding and getting familiar with concepts and properties. They are for developing individual basic skills and tricks for the future. They are the starting point of the journey for expanding our power. But we do not have to go through all of them, since many of them are not worth spending much efforts (See Tips). Therefore we start with questions for fundamental concepts and skills. And then make sure to cover all "Top 100 Liked Questions" and "Top 100 Interview Questions". At this time, we focus on implementation skills.

• Then one "Medium-Hard" problem per day: They are essential problems closer to real daily problems. They are more complexed problems composed of lower level problems with new restrictions, which require us to play with a bunch of basic building blocks flexibly and logically. Make sure to cover all "Top 100 Liked Questions" and "Top 100 Interview Questions". At this time, we focus on solutions skills.

Time Management

Only spend less than 20-30 minutes for generating a solution without implementation. See "How to come up with own solutions".

On one hand, it trains us to get used to the real interview scene.

On the other hand, remember our goal is to get the problem-solving ability through the training. Our time is valuable, we have to spend our time wisely. We do not have to beat every question by our own during the training. No one knows everything when he is growing. The failure guides us to the next success by showing what knowledge do we need to make up, and what skills do we need to spend our time for. See "How to check with other solutions".

How to come up with own solutions

• Grab a piece of white paper.
• 90-10 stratege: 90% time for thinking, 10% time for coding.
• Read the description carefully.
• Find and write down input-output-functionality, know what is the data structure for input.
• Find and write down restrictions.
• Abstract tasks and information to higher level as a conceptuality.
• Determine which group it belongs to.
• Connect to Data Structures, Algorithms and tricks commonly used for the group and estimate if they help to make the tasks easier.
• Manipulate this optional solution with a general example to make sure it works.
• Find other solutions and repeat above steps, and then estimate their time / space complexity.
• Pick one solution and decompose the tasks into sub-tasks according to your chosen tools and mark them down.
• Move on to "How to implement your own solutions".

How to implement your own solutions

• Never start before coming up with own solutions.
• Open a notebook to write it down. It is where we make our hands dirty dealing with details.
• Bottom-up refinement with an example.
• Start with output or return, place them at different locations.
• Around return output to add code blocks for it and comment briefly for how to come up with the return.
• Structure the blocks by cases or loops and comment briefly for each one.
• Add initializations.
• Add guards.
• Double check with the margin cases.
• Verify by drawing test cases and manipulating them with our code. Modify the code to handle the error.
• Use compiler knowledge on codes for possible optimization.
• Optimize (make concise) our codes by combining redundant, but sometimes it reduces readibility.
• Move on to "After implementation".

After implementation

• Copy it to LeetCode and submit.

• Record bugs you couldn't find and do reasoning.

• Check with otherones answers for improvements.

• Move on to "How to check with other solutions".

How to check with other solutions

• Benefit to check with standard solutions and also those in "Discuss" (click "Most Votes" and only for <=2 pages).

• Use "Debugger" to understand the solution step by step if necessary.

• Abstract their tricks and find out how to come up with the tricks from the questions, then extend our tricks set for this topic.

• Evaluate and compare time / space complexity of all solutions.

• Mark the problem number down and review it in the near future.

Tips

• Use filters. E.g., pick "Top 100 Liked Questions".
• Do not work on the problems with 👎 >> 👍. It doesn't mean the problem is "bad", but it is too easy if you have worked out "T-I".
• Only view discussion with 👍 >> 👎.
• Do not connect the problem to a particular problem, but to a general category, because we have limited memory.
• Optimization and conciseness are different.
• Understand the relationship of Data Structures, i.e., Hash Table with don't care values, then use Set.
• Keep in mind the operations can be arranged in different structures, by trade off between readibility and conciseness.
• Conciseness might not always help optimization (which is by estimating complexity and mind-compiling), but reduce readability.
• We will be skilled at conciseness automatically with the time going, but focus on correctness and time / space complexity.
• Do not RUSH! The faster is the slower; The slower is the faster.
• Instead of hacking on solving problems, it benefits from reviewing sloved problems as well as other people's solutions.

Content Table

Note we supply solutions as much as possible for inspiration, but please estimate if it satisfies the question restrictions.

Abbreviations:

• "I" stands for Implementations are supplied, which take the training for coding skills
• "A" stands for Analysis are supplied, which take the training for coming up solutions, and also the overview of all solutions
• "N" stands for no work, either because the analysis and implementation are straight forward, or a slightly alternate to one already seen
• "T-I" stands "Top 100 Interview Questions"
• "T-100" stands "Top 100 Liked Questions"
• "E" stands for "Building blocks", because they are from LeetCode category "Easy"
• "E+" stands for "Requiring changes on Building blocks", also from LeetCode category "Easy"
• "BFS" stands for "Breadth-First Search"
• "BM" stands for "Bit Manipulation"
• "BS" stands for "Binary Search"
• "DC" stands for "Divide and Conquer"
• "DP" stands for "Dynamic Programming"
• "DFS" stands for "Depth-First Search"
• "GR" stands for "Greedy"
• "HT" stands for "Hash Tables"
• "LI" stands for "List"
• "REC" stands for "Recursion"
• "SO" stands for "Sorting"
• "TP" stands for "Two Pointers"
• "* " stands for "Incomplete"
• ":star:" stands for fundamental tasks from LeetCode category "Easy"
• ":star::star:" stands for fundamental tasks from LeetCode category "Easy", but with a little bit change on ":star:"
• ":star2:" stands for fundamental tasks from LeetCode category "Medium"

Array

#	Title	Difficulty	Level	Algorithms	Related	👍 👎	Solutions	Lists
0001	Two Sum	Easy	⭐	HT		>>	N	T-I
0026	Remove Duplicates from Sorted Array	Easy	⭐⭐	TP	27	<<	AI	T-I
0053	Maximum Subarray	Easy	⭐	DC, DP, GR	121	>>	N	T-I
0075	Sort Colors	Medium	🌟	TP		>>	I	T-I
0088	Merge Sorted Array	Easy	⭐	TP	21	<	A	T-I
0121	Best Time to Buy and Sell Stock	Easy	⭐⭐	DC, DP, GR	53	>>	A	T-I
0122	Best Time to Buy and Sell Stock II	Easy	⭐	GR		>>	I	T-I
0136	Single Number	Easy	⭐	HT, SO, BM, MA	242	>>	A	T-I
0169	Majority Element	Easy	⭐	HT, SO, DC, REC		>>	N	T-I
0198	House Robber	Medium	🌟	DP	746	>>	I	T-I
0217	Contains Duplicate	Easy	⭐	SO, Set, HT		>>	AI	T-I
0283	Move Zeroes	Easy	⭐⭐	TP	27	>>	A	T-I
0322	Coin Change	Medium	🌟	DP		>>	I	T-I
0350	Intersection of Two Arrays II	Easy	⭐	TP, BS, HT, SO		>>	N	T-I

String

#	Title	Difficulty	Level	Algorithms	Related	👍 👎	Solutions	Lists
0014*	Longest Common Prefix	Easy	⭐	DC, BS		>>	A	T-I
0020	Valid Parentheses	Easy	⭐	STK		>>	N	T-I
0028	Implement strStr()	Easy	⭐	TP		=	A	T-I
0038	Count and Say	Easy				<	N	T-I
0125	Valid Palindrome	Easy	⭐⭐	TP	304,709	<<	N	T-I
0242	Valid Anagram	Easy	⭐⭐	SO	217	>>	AI	T-I
0344	Reverse String	Easy	⭐	TP, REC	27	>>	AI	T-I
0387	First Unique Character in a String	Easy	⭐	HT, Set		>>	N	T-I

LinkedList

#	Title	Difficulty	Level	Algorithms	Related	👍 👎	Solutions	Lists
0002	Add Two Numbers	Medium	🌟			>>	I	T-I
0019	Odd Even Linked List	Medium	🌟	TP		>>	N	T-I
0021	Merge Two Sorted Lists	Easy	⭐	TP, REC	88	>>	I	T-I
0141	Linked List Cycle	Easy	⭐	TP, Set		>>	AI	T-I
0160	Intersection of Two Linked Lists	Easy	⭐	TP, Set		>>	A	T-I
0206	Reverse Linked List	Easy	⭐	REC		>>	AI	T-I
0234*	Palindrome Linked List	Easy	⭐⭐	TP, REC	206, 876	>>	A	T-I
0237	Delete Node in a Linked List	Easy	⭐			<<	N	T-I
0328	Odd Even Linked List	Medium	🌟	TP		>>	A	T-I

Math & Stack

#	Title	Difficulty	Level	Algorithms	Related	👍 👎	Solutions	Lists
0007	Reverse Integer	Easy	⭐		326	<	N	T-I
0013	Roman to Integer	Easy				<	N	T-I
0069	Sqrt(x)	Easy	⭐	BS, BM	367	<	N	T-I
0070	Climbing Stairs	Easy	⭐	DC, REC	509	>>	AI	T-I
0171	Excel Sheet Column Number	Easy	⭐			>>	N	T-I
0172	Factorial Trailing Zeroes	Easy	⭐			=	N	T-I
0202	Happy Number	Easy	⭐	HT, Set		>>	I	T-I
0204	Count Primes	Easy	⭐	HT		>>	N	T-I
0268	Missing Number	Easy	⭐	BM, SO, HT		=	N	T-I
0326	Power of Three	Easy	⭐			<<	N	T-I

Tree

#	Title	Difficulty	Level	Algorithms	Related	👍 👎	Solutions	Lists
0101	Symmetric Tree	Easy	⭐	BFS, DFS		>>		T-I
0104	Maximum Depth of Binary Tree	Easy	⭐	BFS, DFS, REC		>>		T-I
0108	Convert Sorted Array to Binary Search Tree	Easy	⭐	BFS, DFS		>>		T-I

Non Top 100 Interview Questions

Array

#	Title	Difficulty	Level	Algorithms	Related	👍 👎	Solutions	Lists
0027	Remove Element	Easy	⭐	TP		<<	A
0035	Search Insert Position	Easy	⭐	BS, REC	704	>>	N
0080	Remove Duplicates from Sorted Array II	Medium	🌟	TP	26	>>	AI
0167	Two Sum II - Input array is sorted	Easy	⭐⭐	TP, BS, HT	1	>>	I
0349	Intersection of Two Arrays	Easy		TO, BS, SO, Set	350	<<	N
0448	Find All Numbers Disappeared in an Array	Easy				>>
0704	Binary Search	Easy	⭐	BS, REC	35	>>	N
0724	Find Pivot Index	Easy				>>
0905	Sort Array By Parity	Easy				>>

String

#	Title	Difficulty	Level	Algorithms	Related	👍 👎	Solutions	Lists
0205	Isomorphic Strings	Easy				>>
0290	Word Pattern	Easy				>>
0557	Reverse Words in a String III	Easy	⭐			>>	N
0771	Jewels and Stones	Easy				>>
0844	Backspace String Compare	Easy	⭐	TP, STK		>>	N
1002	Find Common Characters	Easy		HT		>>	N
1047	Remove All Adjacent Duplicates In String	Easy	⭐	STK		>>

LinkedList

#	Title	Difficulty	Level	Algorithms	Related	👍 👎	Solutions	Lists
0083	Remove Duplicates from Sorted List	Easy	⭐⭐	REC	203, 26	>>	I
0203	Remove Linked List Elements	Easy	⭐	REC		>>	AI
0876	Middle of the Linked List	Easy	⭐	TP, LI		>>	AI

Math & Stack

#	Title	Difficulty	Level	Algorithms	Related	👍 👎	Solutions	Lists
0118	Pascal's Triangle	Easy				>>
0119	Pascal's Triangle II	Easy				>>
0155	Min Stack	Easy	⭐	STK		>>
0190	Reverse Bits	Easy	⭐	TP, BM		>>	N
0191	Number of 1 Bits	Easy		BM		>
0232	Implement Queue using Stacks	Easy	⭐	STK, QU		>>
0367	Valid Perfect Square	Easy	⭐	BS	69	>>	N
0374	Guess Number Higher or Lower	Easy	⭐	BS	704	<	N
0389	Find the Difference	Easy	⭐	BM, HT		>>	N
0509	Fibonacci Number	Easy	⭐	REC, DP	70	>	N
0566	Reshape the Matrix	Easy				>
0674	Longest Continuous Increasing Subsequence	Easy	⭐	TP, DP		>>	N
0746	Min Cost Climbing Stairs	Easy	⭐⭐	DP	70	>>	I
0766	Toeplitz Matrix	Easy				>>
0832	Flipping an Image	Easy				>>
0867	Transpose Matrix	Easy				=

Tree

#	Title	Difficulty	Level	Algorithms	Related	👍 👎	Solutions	Lists
0100	Same Tree	Easy				>>
0107	Binary Tree Level Order Traversal II	Easy				>>
0112	Path Sum	Easy				>>
0226	Invert Binary Treee	Easy				>>
0404	Sum of Left Leaves	Easy				>>
0559	Maximum Depth of N-ary Tree	Easy				>>
0572	Subtree of Another Tree	Easy				>>
0700	Search in a Binary Search Tree	Easy				>>
0872	Leaf-Similar Trees	Easy				>>
0938	Range Sum of BST	Easy		REC		>>
0993	Cousins in Binary Tree	Easy				>>

Done before (NOT 👍 >> 👎. Easy if you have worked out "T-I".)

#	Title	Difficulty	Level	Algorithms	Related	👍 👎	Solutions	Lists
0058	Length of Last Word	Easy	⭐			<<	N
0066	Plus One	Easy	⭐			<	N
0219	Contains Duplicate II	Easy				=
0345	Reverse Vowels of a String	Easy				<<
0383	Ransom Note	Easy				>
0412	Fizz Buzz	Easy				<
0414	Third Maximum Number	Easy				<<
0434	Number of Segments in a String	Easy				<<
0485	Max Consecutive Ones	Easy				>
0520	Detect Capital	Easy				>
0521	Longest Uncommon Subsequence I	Easy				<<
0541	Reverse String II	Easy				<<
0551	Student Attendance Record I	Easy				<
0561	Array Partition I	Easy				=
0589	N-ary Tree Preorder Traversal	Easy				>
0605	Can Place Flowers	Easy				>
0657	Robot Return to Origin	Easy				>
0697	Degree of an Array	Easy				=
0705	Design HashSet	Easy				>
0706	Design HashMap	Easy				>>
0709	To Lower Case	Easy	⭐			<<	N
0717	1-bit and 2-bit Characters	Easy				<<
0804	Unique Morse Code Words	Easy				=
0819	Most Common Word	Easy				<<
0824	Goat Latin	Easy				<<
0830	Positions of Large Groups	Easy				>
0859	Buddy Strings	Easy				>
0888	Fair Candy Swap	Easy				>
0893	Groups of Special-Equivalent Strings	Easy				<<
0896	Monotonic Array	Easy				>
0965	Univalued Binary Tree	Easy				>
1160	Find Words That Can Be Formed by Characters	Easy				>
1189	Maximum Number of Balloons	Easy				>
1207	Unique Number of Occurrences	Easy				>

Todo (for up >> down and up > 1000) For practicing topics if needed

#	Title	Difficulty	Level	Algorithms	Related	👍 👎	Solutions	Lists
0009	Palindrome Number	Easy				>>
0067	Add Binary	Easy				>>
0168	Excel Sheet Column Title	Easy				>>
0175	Combine Two Tables	Easy				>>
0231	Power of Two	Easy				>>
0235	Lowest Common Ancestor of a BST	Easy				>>
0278	First Bad Version	Easy				>>
0392	Is Subsequence	Easy				>>
0409	Longest Palindrome	Easy				>>
0415	Add Strings	Easy				>>
0459	Repeated Substring Pattern	Easy				>>
0461	Hamming Distance	Easy				>>
0463	Island Perimeter	Easy				>>
0476	Number Complement	Easy				>>
0501	Find Mode in Binary Search Tree	Easy				>>
0530	Minimum Absolute Difference in BST	Easy				>>
0543	Diameter of Binary Tree	Easy				>>
0617	Merge Two Binary Trees	Easy				>>
0628	Maximum Product of Three Numbers	Easy				>>
0637	Average of Levels in Binary Tree	Easy				>>
0665	Non-decreasing Array	Easy				>>
0653	Two Sum IV - Input is a BST	Easy				>>
0669	Trim a Binary Search Tree	Easy				>>
0680	Valid Palindrome II	Easy				>>
0696	Count Binary Substrings	Easy				>>
0703	Kth Largest Element in a Stream	Easy				>>
0733	Flood Fill	Easy				>>
0796	Rotate String	Easy				>>
0821	Shortest Distance to a Character	Easy				>>
0929	Unique Email Addresses	Easy				>>
0942	DI String Match	Easy				>>
0953	Verifying an Alien Dictionary	Easy				>>
0977	Squares of a Sorted Array	Easy				>>
0997	Find the Town Judge	Easy				>>
1046	Last Stone Weight	Easy				>>
1154	Day of the Year	Easy				>>
1365	How Many Numbers Are Smaller Than the Current Number	Easy				>>