Linked Lists, Trees & Backtracking

Track 2 starts here, and every plan from now on leans on recursion. Linked lists and binary trees are where most people first write a function that calls itself, and backtracking extends the same idea to trying every choice and undoing it.

The five chapters go from linked lists, starting with problems like Reverse Linked List, to binary trees, general trees, backtracking, and recursion with divide and conquer. It suits anyone who wants recursion to feel routine before moving on to grids, dynamic programming and graphs.

292 problemsRating 1151–30185 Chapters

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I. Linked Lists

§1.1 Traverse Linked List

Problem	Rating
1290. Convert Binary Number in a Linked List to Integer	1151
2058. Find the Minimum AND Maximum Number of Nodes Between Critical Points	1311
2181. Merge Nodes in Between Zeros	1333
725. Split Linked List in Parts	Unrated
817. Linked List Components	1429

§1.2 Delete Node

Problem	Rating
203. Remove Linked List Elements	Unrated
3217. Delete Nodes From Linked List Present in Array	1342
83. Remove Duplicates From Sorted List	Unrated
82. Remove Duplicates From Sorted List II	Unrated
237. Delete Node in a Linked List	Unrated
1669. Merge in Between Linked Lists	1428
2487. Remove Nodes From Linked List	1455

§1.3 Insert Node

Problem	Rating
2807. Insert Greatest Common Divisors in Linked List	1279
147. Insertion Sort List	Unrated

§1.4 Reverse Linked List

Problem	Rating
206. Reverse Linked List	Unrated
92. Reverse Linked List II	Unrated
24. Swap Nodes in Pairs	Unrated
25. Reverse Nodes in K Group	Unrated
2074. Reverse Nodes in Even Length Groups	1685

§1.5 Leading and Trailing Pointers

Problem	Rating
19. Remove Nth Node From End of List	Unrated
61. Rotate List	Unrated
1721. Swapping Nodes in a Linked List	1387

§1.6 Fast and Slow Pointers

Problem	Rating
876. Middle of the Linked List	1232
2095. Delete the Middle Node of a Linked List	1324
234. Palindrome Linked List	Unrated
2130. Maximum Twin Sum of a Linked List	1318
143. Reorder List	Unrated
141. Linked List Cycle	Unrated
142. Linked List Cycle II	Unrated
457. Circular Array Loop	Unrated

§1.7 Two Pointers

Problem	Rating
328. Odd Even Linked List	Unrated
86. Partition List	Unrated
160. Intersection of Two Linked Lists	Unrated

§1.8 Merging Linked Lists

Problem	Rating
2. Add Two Numbers	Unrated
445. Add Two Numbers II	Unrated
2816. Double a Number Represented As a Linked List	1394
21. Merge Two Sorted Lists	Unrated

§1.9 Divide-and-Conquer

Problem	Rating
23. Merge K Sorted Lists	Unrated
148. Sort List	Unrated

§1.10 Mixed Practice

Problem	Rating
1171. Remove Zero Sum Consecutive Nodes From Linked List	1782
707. Design Linked List	Unrated
146. Lru Cache	Unrated
460. Lfu Cache	Unrated
432. All Oone Data Structure	Unrated
1206. Design Skiplist	Unrated

§1.11 Miscellaneous

Problem	Rating
138. Copy List with Random Pointer	Unrated
382. Linked List Random Node	Unrated
430. Flatten a Multilevel Doubly Linked List	Unrated

II. Binary Trees

§2.1 Traverse Binary Tree

Problem	Rating
144. Binary Tree Preorder Traversal	Unrated
94. Binary Tree Inorder Traversal	Unrated
145. Binary Tree Postorder Traversal	Unrated
872. Leaf Similar Trees	1288
404. Sum of Left Leaves	Unrated
671. Second Minimum Node in a Binary Tree	Unrated

§2.2 Top-Down DFS (Preorder)

Problem	Rating
104. Maximum Depth of Binary Tree	Unrated
111. Minimum Depth of Binary Tree	Unrated
112. Path Sum	Unrated
129. Sum Root to Leaf Numbers	Unrated
199. Binary Tree Right Side View	Unrated
1448. Count Good Nodes in Binary Tree	1360
1315. Sum of Nodes with Even Valued Grandparent	1427
988. Smallest String Starting From Leaf	1429
1026. Maximum Difference Between Node AND Ancestor	1446
1022. Sum of Root to Leaf Binary Numbers	1462
623. Add One Row to Tree	Unrated
1372. Longest Zigzag Path in a Binary Tree	1713
971. Flip Binary Tree to Match Preorder Traversal	1787

§2.3 Bottom-Up DFS (Postorder)

Problem	Rating
104. Maximum Depth of Binary Tree	Unrated
111. Minimum Depth of Binary Tree	Unrated
965. Univalued Binary Tree	1178
100. Same Tree	Unrated
101. Symmetric Tree	Unrated
951. Flip Equivalent Binary Trees	1477
1379. Find a Corresponding Node of a Binary Tree in a Clone of That Tree	Unrated
110. Balanced Binary Tree	Unrated
226. Invert Binary Tree	Unrated
617. Merge Two Binary Trees	Unrated
2331. Evaluate Boolean Binary Tree	1304
508. Most Frequent Subtree Sum	Unrated
563. Binary Tree Tilt	Unrated
606. Construct String From Binary Tree	Unrated
2265. Count Nodes Equal to Average of Subtree	1473
1026. Maximum Difference Between Node AND Ancestor	1446
3319. K Th Largest Perfect Subtree Size in Binary Tree	1603
1339. Maximum Product of Splitted Binary Tree	1675
1372. Longest Zigzag Path in a Binary Tree	1713
1145. Binary Tree Coloring Game	1741
572. Subtree of Another Tree	Unrated
1530. Number of Good Leaf Nodes Pairs	1746

§2.4 Bottom-Up DFS: Deleting Nodes

Problem	Rating
814. Binary Tree Pruning	1380
1325. Delete Leaves with a Given Value	1407
1110. Delete Nodes AND Return Forest	1511

§2.5 Work on the Way Down and Back Up

Problem	Rating
538. Convert Bst to Greater Tree	Unrated
1038. Binary Search Tree to Greater Sum Tree	1375
865. Smallest Subtree with All the Deepest Nodes	1534
1080. Insufficient Nodes in Root to Leaf Paths	1805

§2.6 Binary Tree Diameter

Problem	Rating
543. Diameter of Binary Tree	Unrated
687. Longest Univalue Path	Unrated
124. Binary Tree Maximum Path Sum	Unrated
2385. Amount of Time for Binary Tree to Be Infected	1711

§2.7 Backtracking

Problem	Rating
257. Binary Tree Paths	Unrated
113. Path Sum II	Unrated
1457. Pseudo Palindromic Paths in a Binary Tree	1405
437. Path Sum III	Unrated

§2.8 Lowest Common Ancestor (LCA)

Problem	Rating
235. Lowest Common Ancestor of a Binary Search Tree	Unrated
236. Lowest Common Ancestor of a Binary Tree	Unrated
1123. Lowest Common Ancestor of Deepest Leaves	1607
2096. Step by Step Directions From a Binary Tree Node to Another	1805

§2.9 Binary Search Tree (BST)

Problem	Rating
700. Search in a Binary Search Tree	Unrated
530. Minimum Absolute Difference in Bst	Unrated
783. Minimum Distance Between Bst Nodes	1303
938. Range Sum of Bst	1335
501. Find Mode in Binary Search Tree	Unrated
230. Kth Smallest Element in a Bst	Unrated
98. Validate Binary Search Tree	Unrated
1305. All Elements in Two Binary Search Trees	1260
99. Recover Binary Search Tree	Unrated
897. Increasing Order Search Tree	1473
2476. Closest Nodes Queries in a Binary Search Tree	1597
653. Two Sum IV Input Is a Bst	Unrated
1373. Maximum Sum Bst in Binary Tree	1914
1932. Merge Bsts to Create Single Bst	2484

§2.10 Building Binary Trees

Problem	Rating
108. Convert Sorted Array to Binary Search Tree	Unrated
998. Maximum Binary Tree II	1498
1008. Construct Binary Search Tree From Preorder Traversal	1563
1382. Balance a Binary Search Tree	1541
2196. Create Binary Tree From Descriptions	1644
105. Construct Binary Tree From Preorder AND Inorder Traversal	Unrated
106. Construct Binary Tree From Inorder AND Postorder Traversal	Unrated
889. Construct Binary Tree From Preorder AND Postorder Traversal	1732
1028. Recover a Tree From Preorder Traversal	1797

§2.11 Inserting and Deleting Nodes

Problem	Rating
701. Insert Into a Binary Search Tree	Unrated
450. Delete Node in a Bst	Unrated
669. Trim a Binary Search Tree	Unrated

§2.12 Tree DP

Problem	Rating
337. House Robber III	Unrated
968. Binary Tree Cameras	2124

§2.13 Binary Tree BFS

Problem	Rating
102. Binary Tree Level Order Traversal	Unrated
103. Binary Tree Zigzag Level Order Traversal	Unrated
107. Binary Tree Level Order Traversal II	Unrated
199. Binary Tree Right Side View	Unrated
513. Find Bottom Left Tree Value	Unrated
515. Find Largest Value in Each Tree Row	Unrated
637. Average of Levels in Binary Tree	Unrated
1161. Maximum Level Sum of a Binary Tree	1250
993. Cousins in Binary Tree	1288
2583. Kth Largest Sum in a Binary Tree	1374
1302. Deepest Leaves Sum	1388
2415. Reverse Odd Levels of Binary Tree	1431
1609. Even Odd Tree	1438
623. Add One Row to Tree	Unrated
2471. Minimum Number of Operations to Sort a Binary Tree by Level	1635
2641. Cousins in Binary Tree II	1677
919. Complete Binary Tree Inserter	1691
958. Check Completeness of a Binary Tree	1703
863. All Nodes Distance K in Binary Tree	1663
662. Maximum Width of Binary Tree	Unrated

§2.14 Linked Lists and Binary Trees

Problem	Rating
114. Flatten Binary Tree to Linked List	Unrated
1367. Linked List in Binary Tree	1650
109. Convert Sorted List to Binary Search Tree	Unrated
116. Populating Next Right Pointers in Each Node	Unrated
117. Populating Next Right Pointers in Each Node II	Unrated

§2.15 N-ary Trees

Problem	Rating
589. N Ary Tree Preorder Traversal	Unrated
590. N Ary Tree Postorder Traversal	Unrated
559. Maximum Depth of N Ary Tree	Unrated
429. N Ary Tree Level Order Traversal	Unrated
427. Construct Quad Tree	Unrated
558. Logical OR of Two Binary Grids Represented As Quad Trees	Unrated

§2.16 Miscellaneous

Problem	Rating
1261. Find Elements in a Contaminated Binary Tree	1440
1104. Path in Zigzag Labelled Binary Tree	1545
987. Vertical Order Traversal of a Binary Tree	1676
655. Print Binary Tree	Unrated
979. Distribute Coins in Binary Tree	1709
222. Count Complete Tree Nodes	Unrated
297. Serialize AND Deserialize Binary Tree	Unrated
449. Serialize AND Deserialize Bst	Unrated
331. Verify Preorder Serialization of a Binary Tree	Unrated
652. Find Duplicate Subtrees	Unrated
173. Binary Search Tree Iterator	Unrated
2049. Count Nodes with the Highest Score	1912
2673. Make Costs of Paths Equal in a Binary Tree	1917
2509. Cycle Length Queries in a Tree	1948
2458. Height of Binary Tree After Subtree Removal Queries	2299

III. General Trees

§3.1 General Tree Traversal

Problem	Rating
2368. Reachable Nodes with Restrictions	1477
1466. Reorder Routes to Make All Paths Lead to the City Zero	1634

§3.2 Top-Down Tree DFS

Problem	Rating
1376. Time Needed to Inform All Employees	1561
3528. Unit Conversion I	1580
1443. Minimum Time to Collect All Apples in a Tree	1683
1377. Frog Position After T Seconds	1824
3067. Count Pairs of Connectable Servers in a Weighted Tree Network	1909
3372. Maximize the Number of Target Nodes After Connecting Trees I	1927
2467. Most Profitable Path in a Tree	2053
3373. Maximize the Number of Target Nodes After Connecting Trees II	2162
1766. Tree of Coprimes	2232
2791. Count Paths That Can Form a Palindrome in a Tree	2677

§3.3 Bottom-Up Tree DFS

Problem	Rating
690. Employee Importance	Unrated
3249. Count the Number of Good Nodes	1566
1519. Number of Nodes in the Sub Tree with the Same Label	1809
3558. Number of Ways to Assign Edge Weights I	1845
3593. Minimum Increments to Equalize Leaf Paths	1959
2872. Maximum Number of K Divisible Components	1968
2477. Minimum Fuel Cost to Report to the Capital	2012
2973. Find Number of Coins to Place in Tree Nodes	2277
2440. Create Components with Same Value	2460

§3.4 Work on the Way Down and Back Up

Problem	Rating
3593. Minimum Increments to Equalize Leaf Paths	1959
3331. Find Subtree Sizes After Changes	2046

§3.5 Tree Diameter

Problem	Rating
2246. Longest Path with Different Adjacent Characters	2126
3203. Find Minimum Diameter After Merging Two Trees	2266
1617. Count Subtrees with Max Distance Between Cities	2309
2538. Difference Between Maximum AND Minimum Price Sum	2398

§3.6 Topological Sort on Trees

Problem	Rating
310. Minimum Height Trees	Unrated
2603. Collect Coins in a Tree	2712

§3.7 DFS Timestamps (Euler Tour)

Problem	Rating
3515. Shortest Path in a Weighted Tree	2312
2322. Minimum Score After Removals on a Tree	2392
3327. Check If DFS Strings Are Palindromes	2454
3590. Kth Smallest Path XOR Sum	2646

§3.8 Lowest Common Ancestor (LCA) and Binary Lifting

Problem	Rating
1483. Kth Ancestor of a Tree Node	2115
3559. Number of Ways to Assign Edge Weights II	2146
3553. Minimum Weighted Subgraph with the Required Paths II	2411
3585. Find Weighted Median Node in Tree	2429
2846. Minimum Edge Weight Equilibrium Queries in a Tree	2508
2646. Minimize the Total Price of the Trips	2238

§3.9 Sliding Window on Trees

Problem	Rating
3425. Longest Special Path	2435
3486. Longest Special Path II	2925

§3.10 Small-to-Large Merging on Trees

Problem	Rating
2003. Smallest Missing Genetic Value in Each Subtree	2415
3590. Kth Smallest Path XOR Sum	2646
3575. Maximum Good Subtree Score	2360

§3.11 Centroid Decomposition

Problem	Rating
3372. Maximize the Number of Target Nodes After Connecting Trees I	1927

§3.12 Miscellaneous

Problem	Rating
2867. Count Valid Paths in a Tree	2428
2421. Number of Good Paths	2445
1719. Number of Ways to Reconstruct a Tree	3018

IV. Backtracking

§4.1 Introductory Backtracking

Problem	Rating
17. Letter Combinations of a Phone Number	Unrated

§4.2 Subset Backtracking

Problem	Rating
78. Subsets	Unrated
1863. Sum of All Subset XOR Totals	1372
784. Letter Case Permutation	1342
3566. Partition Array Into Two Equal Product Subsets	1459
2044. Count Number of Maximum Bitwise OR Subsets	1568
2397. Maximum Rows Covered by Columns	1719
1239. Maximum Length of a Concatenated String with Unique Characters	1720
2212. Maximum Points in an Archery Competition	1869
1255. Maximum Score Words Formed by Letters	1882
2151. Maximum Good People Based on Statements	1980
2597. The Number of Beautiful Subsets	2023
2959. Number of Possible Sets of Closing Branches	2077
1601. Maximum Number of Achievable Transfer Requests	2119
1617. Count Subtrees with Max Distance Between Cities	2309

§4.3 Partition Backtracking

Problem	Rating
131. Palindrome Partitioning	Unrated
2698. Find the Punishment Number of an Integer	1679
1593. Split a String Into the Max Number of Unique Substrings	1740
1849. Splitting a String Into Descending Consecutive Values	1747
93. Restore Ip Addresses	Unrated
140. Word Break II	Unrated

§4.4 Combination Backtracking

Problem	Rating
77. Combinations	Unrated
216. Combination Sum III	Unrated
1286. Iterator for Combination	1591
22. Generate Parentheses	Unrated
301. Remove Invalid Parentheses	Unrated

§4.5 Permutation Backtracking

Problem	Rating
46. Permutations	Unrated
3376. Minimum Time to Break Locks I	1793
51. N Queens	Unrated
52. N Queens II	Unrated
2850. Minimum Moves to Spread Stones Over Grid	2001
1718. Construct the Lexicographically Largest Valid Sequence	2080
1307. Verbal Arithmetic Puzzle	2250
37. Sudoku Solver	Unrated

§4.6 Backtracking with Duplicate Elements

Problem	Rating
90. Subsets II	Unrated
40. Combination Sum II	Unrated
491. Non Decreasing Subsequences	Unrated
47. Permutations II	Unrated
1079. Letter Tile Possibilities	1741
3646. Next Special Palindrome Number	2445
2014. Longest Subsequence Repeated K Times	2558

§4.7 Backtracking Search

Problem	Rating
3211. Generate Binary Strings Without Adjacent Zeros	1353
967. Numbers with Same Consecutive Differences	1433
2094. Finding 3 Digit Even Numbers	1455
1415. The K Th Lexicographical String of All Happy Strings of Length N	1576
1219. Path with Maximum Gold	1663
79. Word Search	Unrated
980. Unique Paths III	1830
2002. Maximum Product of the Length of Two Palindromic Subsequences	1869
1255. Maximum Score Words Formed by Letters	1882
3669. Balanced K Factor Decomposition	1917
473. Matchsticks to Square	Unrated
212. Word Search II	Unrated
638. Shopping Offers	Unrated
126. Word Ladder II	Unrated
691. Stickers to Spell Word	Unrated
2056. Number of Valid Move Combinations on Chessboard	2611
2386. Find the K Sum of an Array	2648
3509. Maximum Product of Subsequences with an Alternating Sum Equal to K	2703
488. Zuma Game	Unrated

§4.8 Meet in the Middle

Problem	Rating
805. Split Array with Same Average	1983
494. Target Sum	Unrated
3566. Partition Array Into Two Equal Product Subsets	1459
1755. Closest Subsequence Sum	2364
956. Tallest Billboard	2381
2035. Partition Array Into Two Arrays to Minimize Sum Difference	2490
3267. Count Almost Equal Pairs II	2545

V. Recursion & Divide-and-Conquer

V. Recursion and Divide-and-Conquer

Problem	Rating
3537. Fill a Special Grid	1542
912. Sort an Array	Unrated
880. Decoded String at Index	2011
3307. Find the K Th Character in String Game II	2232
1545. Find Kth Bit in Nth Binary String	1479
3614. Process String with Special Operations II	2011
932. Beautiful Array	2294

It is the first plan on Track 2; the route suggests it after Strings & Automata, but you can start here directly by picking it when you sign your study contract.

Ratings come from zerotrac's LeetCode contest ratings. Some problems have no zerotrac rating and are marked Unrated.

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