Two Pointers

15. 3sum

16. 3sum closet

18. 4sum

643. Maximim average subarray i

26. Remove duplicates from sorted array

581. Shortest unsorted continuous subarray

136. Single number

75. Sort colors

977. Squares of a sorted array

1. Two-sum

Matrix

1559. Detect cycles in 2d grid

733. Flood fill

463. Island perimeter

1254. Number of closed islands

200. Number of islands

867. Transpose matrix

Find number of distinct island in a 2d matrix

Sliding Window

3. Longest Substring Without Repeating Characters

187. Repeated dna sequence

209. Minimum size subarray sum

424. Logest repeating character replacement

121. Best time to buy and sell stock

2269. Find the k beatu of a number

844. Backspace string compare

Fast & Slow Pointers

141. Linked list cycle

202. Happy number

234. Palindrome linkedlist

876. Middle of the linked list

287. Find the duplicate number

Binary Tree

94. Binary Tree Inorder Traversal

98. Validate binary search tree

100. Same Tree

101. Symmetric Tree

102. Binary Tree Level Order Traversal

104. Maximum depth of binary tree

107. Binary Tree Level Order Traversal II

108. Convert sorted array to binary search tree

110. Balanced Binary Tree

112. Path Sum

114. Flatten Binary Tree to Linked List

124. Binary Tree Maximum Path Sum

144. Binary Tree Preorder Traversal

145. Binary Tree Postorder Traversal

199. Binary tree right side view

208. Implement Trie (Prefix Tree)

226. Invert binary tree

235. Lowest Common Ancestor of a Binary Search Tree

236. Lowest Common Ancestor of a Binary Tree

543. Diameter of binary tree

617. Merge Two Binary Trees