Data Structures:

78 Subset(Also Bitset)
94 Binary Tree Inorder Traversal (Inorder Tree Traversal Iterative Approach)
98 Validate Binary Tree
102 Level Order Traversal
114 Binary Tree To Linked List (Reversed Post Order Traversal)
138 Deep Copy List with Random Pointers (Hashmap to save past pointer & future pointers)
347 TopKFrequencies (Priority Queue!, complexity analysis)
341 Flatten Nested List Iterator (stack)

Arithmatics:

29 Divide2Integers

Math:

4 Median of Two Sorted Arrays (Representation of Median, Gap indexing, Binary search, Partitioning)
166 Fraction to Decimals (Simulate Division)

Sorting, Searching:

33 34 (Revised Binary Search)
39 Combination Sum (Depth First Search with Backtracking)
79 Word Search (DFS)
148 Sort List (Merge Sort, Quicksort)
162 Find Peak Element (Variant of Binary Search)
179 Largest Number (Custom Sorting Comparator)
324 Wiggle Sort (Dutch Flag Partitioning Problem, Index Mapping, Quick Select HARD)
127 Word Ladder (BFS, adjacency list)
207 Course Schedule (DFS Graph Cycle Detection, BFS topological sort)
210 Course Schedule II (DFS Graph Cycle Detection, BFS topological sort)
230 Kth Smallest Element In BST (.. is the order of access in INORDER traversal)

Backtracking:

131 Palindrome Partitioning (Memoization, DP)
78 Subset

String:

227 Basic Calculator II

Dynamic Programming:

416 Partition Equal Subset Sum
55 Jump Game (Recusion -> Top Down Dp -> Bottom Up Dp -> Greedy | Good Article)

464 Can Win (Relate to Combination Sum, also Backtracking, with memorization)

Index Manipulation:

48 Rotate Image (Requires careful on paper analysis)
54 Spiral Matrix (Non-square matrix, requires second pass)
73 Set Matrix Zeroes O(1) Space Solution

Hashing:

49 Group Anagrams (A nice orderless hashing function for string using prime numbers)

Two Pointer (Floyd Tortoise and Hare)：

287 Find Duplicate Number
142 Linked List Cycle

Trie

208 Implement Trie

Other

41 First Missing Positive
42 Trapping Rainwater (Dynamic Programming, Two Pointers)