README.md

Code practice link

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Templates

Difficulty	Marks	Name	link	Notes
🟢	✅❗❓❌		Leetcode
🟠	✅❗❓❌		Leetcode
🔴	✅❗❓❌		Leetcode

Markers:

• ✅/✔️ For finished
• 👷 For working in progress
• ❌ For unfinished due to complicity or challenging
• ❗For marking good questions
• ❓with doubt or unfinished
• Strikedthrough For not bothering (e.g. too easy)
• MARK in code for paying extra attention

Difficulties:

• 🟢(Easy)
• 🟠(Medium)
• 🔴(Hard) 🤨

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Kotlin

Since I use kotlin, I have created some convenient utility functions, mostly for testing/debugging purpose. (I might just create a branch with just the utility functions, however.. )

Note: To enable assertion for kotlin, pass in -ea as jvm launch argument, see this link

Other than those listed below, check out _util.kt for more

1. Run timed tests on a list of method variable. Sample usage checkout this Update: I noticed that this method may not be accurate, not sure of the cause

@OptIn(ExperimentalTime::class)
inline fun<reified T : KCallable<*>, reified R> List<T>.runTimedTests(
    testName: String? = Thread.currentThread().stackTrace.getOrNull(1)?.className?.split("$", limit = 0)?.get(0),
    printTime: Boolean = true,
    block: T.()->R
) {
    forEachIndexed { index, it ->
        val timeValue = measureTimedValue{ it.block() }
        val prefix = if (testName.isNullOrEmpty()) "" else "${testName}."
        if (printTime) println("execution for ${prefix}${it.name} finished, took ${timeValue.duration.inWholeNanoseconds} Nanoseconds")
    }
}

Sample print out:

execution for _2022._05.Leetcode_15.MY1.threeSum finished, took 476348 Nanoseconds
execution for _2022._05.Leetcode_15.MY1.threeSum finished, took 423606 Nanoseconds
execution for _2022._05.Leetcode_15.MY2.threeSum finished, took 435733 Nanoseconds

2. Assertion (assertEqual)

test() assertEqual "expected" // for single answer
test().assertEqual("possible_output1", "possible_output2") // for multiple correct answers 

3. Build-in data types
   1. Node (typealias IntNode = Node<Int>) and some built-in functions, e.g:
      1. fun <T> linkedNodesOf(vararg nodes: T) : Node<T> usage:linkedNodesOf(1, 2, 3) to create a linked nodes like 1 -> 2 -> 3
      2. fun <T> Node<T>.toList() : List<T> usage: linkedNodesOf(1, 2, 3).toList() gets: [1,2,3]
      3. fun <T, R> Node<T>.mapValue(transform: ((T)->R)) : List<R> usage: linkedNodesOf(1, 2, 3).mapValue{ it * 2 } gets [2,4,6]
      4. others see this file: Node

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202205

20220528

youngyangyang04/leetcode-master/0001.两数之和.md · GitHub

Marks	link	Notes
🟢✅	1. Two Sum
🟢✅	342. Valid Anagram
🟢✅	349. Intersection of Two Arrays
🟢	383. Ransom Note
🟢✅	49. Group Anagrams

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20220529, 20220601

youngyangyang04/leetcode-master/0005.最长回文子串.md · GitHub

Marks	link	Notes
🟠✅	647. Palindromic Substrings
🟠❗❓	5. Longest Palindromic Substring

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20220602, 20220605, 20220606

youngyangyang04/leetcode-master/0005.最长回文子串.md · GitHub

Difficulty	Marks	link	Notes
Medium	✅❗	5. Longest Palindromic Substring [leetcode] [git]
Medium	✅	15. 3Sum 💃💃💃 [leetcode] [git]	My4 not done

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20220606

Difficulty	Marks	Name	link	Notes
Easy	🟠✅
Medium	🟢✅	17. Letter Combinations of a Phone Number	Leetcode Github	Didn't read the sample code since it didn't take me too much thinking to solve the question, but it might worth a read
Medium	Skipped for now	18. 4Sum 💃💃💃💃	Leetcode Github
Medium	🟠❓	19. Remove Nth Node From End of List	Leetcode Github

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20220607 labuladong

Difficulty	Marks	Name	link	Notes
Easy	🟢✅	21. Merge Two Sorted Lists	Leetcode labuladong	Sample1 is interesting (recursion)
Easy	🟢✅	141. Linked List Cycle	Leetcode
Medium	❗	142. Linked List Cycle II	Leetcode labuladong

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20220608,20220614 labuladong

Difficulty	Marks	Name	link	Notes
Hard	🟢✅	23. Merge k Sorted Lists	Leetcode labuladong	Need to read the sample code
Easy	❗	160. Intersection of Two Linked Lists	Leetcode labuladong	Sample1 is interesting (recursion)

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20220614,20220615

labuladong

Difficulty	Marks	Name	link	Notes
Easy	🟢	876. Middle of the Linked List	Leetcode	Faster than 100% submission 🟢🟢
Medium	❗	92. Reverse Linked List II	Leetcode	My2
Easy	🟠✅	206. Reverse Linked List	Leetcode	Need to check sample and read this and read S1

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20220615,20220615

Difficulty	Marks	Name	link	Notes
Hard	🔴❗❗❓	25. Reverse Nodes in k-Group	Leetcode

TODO:

• https://labuladong.github.io/algo/2/17/18/

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20220616

Difficulty	Marks	Name	link	Notes
🟢	❌	234. Palindrome Linked List	Leetcode
🟢	✅	26. Remove Duplicates from Sorted Array	Leetcode
🟢	✅	27. Remove Element	Leetcode
🟢	✅	83. Remove Duplicates from Sorted List	Leetcode

TODO: Leetcode_710

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20220617

Difficulty	Marks	Name	link	Notes
🟠	✅🟢	167. Two Sum II - Input Array Is Sorted	Leetcode
🟢	✅	283. Move Zeroes	Leetcode

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20220618

Difficulty	Marks	Name	link	Notes
🟢	✅	344. Reverse String	Leetcode
🟢	✅	303. Range Sum Query - Immutable	Leetcode
🟠	✅	304. Range Sum Query 2D - Immutable	Leetcode

Leetcode 370: Range Addition

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20220619

Difficulty		Marks		Name	link	Notes
🟠		❓		370: Range Addition	Leetcode	Put off for now
🟠		✅🟢		1094. Car Pooling	Leetcode
🟠		❓		1109. Corporate Flight Bookings	Leetcode
🟠		❓		48. Rotate Image	Leetcode

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20220620

Difficulty	Marks	Name	link	Notes
🟠	✅	54. Spiral Matrix	Leetcode

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20220621

Difficulty	Marks	Name	link	Notes
🟠	❓	59. Spiral Matrix II	Leetcode
🟠	✅❓	3. Longest Substring Without Repeating Characters	Leetcode	TODO: read explanation

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20220627,20200630

Difficulty	Marks	Name	link	Notes
🔴		76. Minimum Window Substring	Leetcode
🟠		34. Find First and Last Position of Element in Sorted Array	Leetcode
🟠		1109. Corporate Flight Bookings	Leetcode

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20220701

Difficulty	Marks	Name	link	Notes
🟠	✅	11. Container With Most Water	Leetcode
🔴	✅❗❓❌	42. Trapping Rain Water	Leetcode

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20220702

Difficulty	Marks	Name	link	Notes
🟠	✅	238. Product of Array Except Self	Leetcode
🟢	✅	217. Contains Duplicate	Leetcode
🟠	✅	347. Top K Frequent Elements	Leetcode	1. Heatmap, 2. bucket list/sort
🟢	✅	125. Valid Palindrome	Leetcode
🟠	✅	271. Encode and Decode Strings	Lintcode_659

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20220703

Difficulty	Marks	Name	link	Notes
🟠	✅	128. Longest Consecutive Sequence	Leetcode
🟢	✅	121. Best Time to Buy and Sell Stock	Leetcode
🟠	✅❓	424. Longest Repeating Character Replacement	Leetcode
🟠	✅	567. Permutation in String	Leetcode

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20220704

Difficulty	Marks	Name	link	Notes
🔴	✅❗❓❌	239. Sliding Window Maximum	Leetcode
🟢	✅	20. Valid Parentheses	Leetcode
🟠	✅❗	155. Min Stack	Leetcode	The use of miniStack in S1 and S2

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20220705

Difficulty	Marks	Name	link	Notes
🟠	✅	150. Evaluate Reverse Polish Notation	Leetcode
🟠	✅❓	22. Generate Parentheses	Leetcode
🟢	✅❗❓❌	496. Next Greater Element I	Leetcode

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20220724

Difficulty	Marks	Name	link	Notes
🟠	✅	739. Daily Temperatures	Leetcode
🟠	✅	853. Car Fleet	Leetcode

                                        |            |

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20220725

Difficulty	Marks	Name	link	Notes
🔴	❌	84. Largest Rectangle in Histogram	Leetcode
🟢	✅	704. Binary Search	Leetcode
🟠	✅	74. Search a 2D Matrix	Leetcode
🟠	✅❗	875. Koko Eating Bananas	Leetcode
🟠	❌	33. Search in Rotated Sorted Array	Leetcode
🟢	✅❗❓❌		Leetcode
🟠	✅❗❓❌		Leetcode
🔴	✅❗❓❌		Leetcode

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20220726

Difficulty	Marks	Name	link	Notes
🟠	✅❗❓❌	143. Reorder List	Leetcode
🟢	✅❗❓❌		Leetcode
🟠	✅❗❓❌		Leetcode
🔴	✅❗❓❌		Leetcode

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20220727

Difficulty	Marks	Name	link	Notes
🟠	✅	138. Copy List with Random Pointer	Leetcode

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2022078

Difficulty	Marks	Name	link	Notes
🟠	✅	2. Add Two Numbers	Leetcode
🟠	❌	287. Find the Duplicate Number	Leetcode
🟠	❌	146. LRU Cache	Leetcode

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20220729

Difficulty	Marks	Name	link	Notes
🟢	✅	226. Invert Binary Tree	Leetcode
🟢	✅❗	104. Maximum Depth of Binary Tree	Leetcode
🟢	✅	110. Balanced Binary Tree	Leetcode
🟢	✅	100. Same Tree	Leetcode
🟢	✅❗	572. Subtree of Another Tree	Leetcode

• 572: can use a utility method isSameTree to check or need to be careful (see solution)

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20220730

Difficulty	Marks	Name	link	Notes
🟢	✅	235. Lowest Common Ancestor of a Binary Search Tree	Leetcode
🟢	✅❗❓❌	703. Kth Largest Element in a Stream	Leetcode
🟢	✅	1046. Last Stone Weight	Leetcode
🟢	✅	70. Climbing Stairs	Leetcode
🟢	✅	746. Min Cost Climbing Stairs	Leetcode
🟢	✅❗❓❌		Leetcode
🟠	✅❗❓❌		Leetcode
🔴	✅❗❓❌		Leetcode

• 235: Binary search tree: left <= val <= right
• 746: Build an array dp where dp[i] is the minimum cost to climb to the top starting from the ith staircase.

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20220731

Difficulty	Marks	Name	link	Notes
🟢	✅	136. Single Number	Leetcode
🟢	✅❗	191. Number of 1 Bits	Leetcode
🟢	❌	338. Counting Bits	Leetcode
🟢	✅	190. Reverse Bits	Leetcode
🟢	✅❗❓❌	268. Missing Number	Leetcode

• 191: use mod n = n % 2

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20220802

Difficulty	Marks	Name	link	Notes
🟠	✅	208. Implement Trie (Prefix Tree)	Leetcode
🟠	❓	211. Design Add and Search Words Data Structure	Leetcode
🟠	✅	981. Time Based Key-Value Store	Leetcode
🟠	✅	973. K Closest Points to Origin	Leetcode
🟠	✅	215. Kth Largest Element in an Array	Leetcode
🟠	✅	78. Subsets	Leetcode	try to solve with single res: ArrayList
🟠	✅❗❓❌	39. Combination Sum	Leetcode	S1

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20220803

Difficulty	Marks	Name	link	Notes
🟠	✅	46. Permutations	Leetcode
🟠	✅❗❓❌	90. Subsets II	Leetcode

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20220804

Difficulty	Marks	Name	link	Notes
🟠	❓	40. Combination Sum II	Leetcode
🟠	✅❗	200. Number of Islands	Leetcode

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20220805

Difficulty	Marks	Name	link	Notes
🟠	✅❗❓❌	695. Max Area of Island	Leetcode
🟠	✅❗❓❌	133. Clone Graph	Leetcode

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20220806

Difficulty	Marks	Name	link	Notes
🟠	✅	417. Pacific Atlantic Water Flow	Leetcode	revert: ocean to land
🟠	✅❗❓❌	130. Surrounded Regions	Leetcode

Note:

• 130: reverse thinking, thanks all non-surrounded region to T and change all O to X then change Ts back to O

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20220807

Difficulty	Marks	Name	link	Notes
🟠	✅❗	198. House Robber	Leetcode

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20220808

Difficulty	Marks	Name	link	Notes
🟠	✅❗❓❌	322. Coin Change	Leetcode
🟠	✅❗❓❌	91. Decode Ways	Leetcode

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20220810

Difficulty	Marks	Name	link	Notes
🟠	✅	36. Valid Sudoku	Leetcode
🟠	✅❗❓❌	53. Maximum Subarray	Leetcode

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20220811

Difficulty	Marks	Name	link	Notes
🟠	✅❗	134. Gas Station	Leetcode
🟠	✅❓❓	55. Jump Game	Leetcode
🟠	✅❓	45. Jump Game II	Leetcode

Notes:

• 55: M4
• 134: S1

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20220812

Difficulty	Marks	Name	link	Notes
🟠	✅	846. Hand of Straights	Leetcode
🟠	✅❓	57. Insert Interval	Leetcode

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20220813

Difficulty	Marks	Name	link	Notes
🟠	✅❗❓❌	102. Binary Tree Level Order Traversal	Leetcode

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20220814

Difficulty	Marks	Name	link	Notes
🟠	✅	199. Binary Tree Right Side View	Leetcode
🟠	✅	1448. Count Good Nodes in Binary Tree	Leetcode
🟠	✅	98. Validate Binary Search Tree	Leetcode
🟠	✅❗	230. Kth Smallest Element in a BST	Leetcode
🟠	✅❗❗❗	105. Construct Binary Tree from Preorder and Inorder Traversal	Leetcode
🟠	✅❗❓❌	211. Design Add and Search Words Data Structure	Leetcode
🟠	✅❓	621. Task Scheduler	Leetcode

Notes:

• bfs - order level traversal
• 98: binary search tree - sorted order
• 230: solution

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20220815

Difficulty	Marks	Name	link	Notes
🟢	✅❗❓❌		Leetcode
🟠	✅❓	355. Design Twitter	Leetcode
🟠	✅	332. Reconstruct Itinerary	Leetcode
🟠	✅❓	1584. Min Cost to Connect All Points	Leetcode

Notes:

• 1584: Read S2 and link

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2022081

Difficulty	Marks	Name	link	Notes
🟢	✅❗❓❌		Leetcode
🟠	✅	143. Reorder List	Leetcode
🟠	✅	146. LRU Cache	Leetcode
🟠	✅	23. Merge k Sorted Lists	Leetcode
🔴	✅	25. Reverse Nodes in k-Group	Leetcode
🟠		211. Design Add and Search Words Data Structure	Leetcode
🟠	❗❓❌	4. Median of Two Sorted Arrays	Leetcode
🟠	✅	130. Surrounded Regions	Leetcode

Notes:!!

• 143: slow -> 0 and fast -> 1, fast jumps two steps while slow jumps one, when fast became null slow is at the end of the first half

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202208

Difficulty	Marks	Name	link
🟠	✅❗❓❌	743. Network Delay Time	Leetcode
🟠	✅❗❓❌	295. Find Median from Data Stream	Leetcode

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20220824

Difficulty	Marks	Name	link
🟠	✅	79. Word Search	Leetcode

---

20220825

Difficulty	Marks	Name	link
🟠	❌	131. Palindrome Partitioning	Leetcode
🟠	✅❗❓❌	17. Letter Combinations of a Phone Number	Leetcode

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20220826

Difficulty	Marks	Name	link
🟠	✅	663 · Walls and Gates	Leetcode
🟠	✅	207. Course Schedule	Leetcode

• 207: S1

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20220827

Difficulty	Marks	Name	link
🟠	✅❗❓❌	684. Redundant Connection	[Leetcode](// https://leetcode.com/problems/redundant-connection/)

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20220828

Difficulty	Marks	Name	link
🟠	✅	2385. Amount of Time for Binary Tree to Be Infected	Leetcode

• 2385: S1

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20220829

Difficulty	Marks	Name	link
🔴	✅❗❓❌	4. Median of Two Sorted Arrays	Leetcode

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20220907

Difficulty	Marks	Name	link
🟠	✅❗❓❌	8. String to Integer (atoi)	Leetcode
🟠	✅❗❓❌	56. Merge Intervals	Leetcode

• 8: stupid ass question, not worth looking at

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20220908

Difficulty	Marks	Name	link
🟠	❌	435. Non-overlapping Intervals	Leetcode
🟠	✅❗❓❌	Lintcode 919 · Meeting Rooms II	Leetcode youtube

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20220911

Difficulty	Marks	Name	link
🟠	✅		Leetcode
🟠	✅	73. Set Matrix Zeroes	Leetcode
🟠	✅	50. Pow(x, n)	Leetcode
🟠	❌	371. Sum of Two Integers	Leetcode
🟢	✅	202. Happy Number	Leetcode
🟢	✅	66. Plus One	Leetcode

---

20220912

Difficulty	Marks	Name	link
🟠	✅	338. Counting Bits	Leetcode
🟠	✅❗❓❌	91. Decode Ways	Leetcode

---

20220914

Difficulty	Marks	Name	link
🟠	✅❗❓❌	213. House Robber II	Leetcode

• 213: run house robber 1 two times with either first house included or the last

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20220916

Difficulty	Marks	Name	link
🟠	✅❗❓❌	322. Coin Change	Leetcode

---

20220923

Diff	Marks	Name	link
🟠	✅❗	743. Network Delay Time	Leetcode
🟠	✅	209. Minimum Size Subarray Sum	Leetcode
🟠	✅❗❓❌		Leetcode
🟠	✅❗❓❌		Leetcode
🟠	✅❗❓❌		Leetcode
🟢	✅❗❓❌		Leetcode
🟢	✅❗❓❌	977. Squares of a Sorted Array	Leetcode
🔴	✅❗❓❌		Leetcode

• 209: sliding window (left/right pointers)

---

202209

Diff	Marks	Name	link
🟠	✅❗❓❌		Leetcode
🟠	✅❗❓❌		Leetcode
🟠	✅❗❓❌		Leetcode
🟠	✅❗❓❌		Leetcode
🟢	✅❗❓❌		Leetcode
🔴	✅❗❓❌		Leetcode

---

Read List

• Union-Find/Disjoint Set

TODO:

• Quick sort
• Quick select
• depth first search

Binary search tree

• when there are multiple same value (not distinct), how to get the left most index of the given value (or right)
• l < r vs l <= r

backtrack / dfs

• you can go either way
  • starting from position 0, and for each layer down the line you can either add or not add the next element (Leetcode_46), or
  • starting form all the element including, you can have any many children as the element, and for each child, consider the corresponding element removed (Leetcode_90)

References:

• LeetCode Top 100 Problem Selection | Step-by-step Data Science
• GitHub - youngyangyang04/leetcode-master: 🚀 《代码随想录》LeetCode 刷题攻略：200道经典题目刷题顺序
• LABULADONG 的算法网站

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To-Do list

• https://leetcode.com/problems/number-of-connected-components-in-an-undirected-graph/
• https://leetcode.com/problems/graph-valid-tree/
• https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/
• https://leetcode.com/problems/palindrome-partitioning/submissions/
• https://leetcode.cn/problems/construct-binary-tree-from-preorder-and-inorder-traversal/