A friend gave me this, so I won’t make it public.

Leetcode Interview High-Frequency Problems Summary

Inspired by: https://zhuanlan.zhihu.com/p/349940945

This article contains 200+ problems, with derivatives about 250+. Mostly medium difficulty, few easy, some hard. Enough for most algorithm interviews.

Sort:

• Basics: Quick Sort & Merge Sort implementation, Time Complexity, In-place, Stability
• Entry: Leetcode 148, 56, 27
• Advanced: Leetcode 179, 75

Selection:

• Quick Selection, Heapsort. Know the trade-offs, know the corresponding C++ STL methods. Hand-writing quick selection is sometimes necessary.
  • Leetcode 215, 347, 2099 (clear progression)
  • Leetcode 4. Median of Two Sorted Arrays

Linked List:

• Basics: How to implement and traverse a linked list. Head/tail insert/delete are O(1), finding any element is O(N)
• Entry:
  • Leetcode 206. Reverse Linked List
  • Leetcode 876. Middle of the Linked List

Fast/slow pointers and list reversal are fundamental to almost all linked list problems. Reversal code is short - memorize it.

• Advanced:
  • Leetcode 160. Intersection of Two Linked Lists
  • Leetcode 141. Linked List Cycle (and Cycle II)
  • Leetcode 92. Reverse Linked List II
  • Leetcode 328. Odd Even Linked List

Heap, Dictionary (Set, Map, Hashmap), Stack, Queue:

• Basics: Read Introduction to Algorithms relevant chapters.
• Queue:
  • Leetcode 225, 346, 281, 1429, 54, 362
• Stack:
  • Leetcode 155, 232, 150, 224, 20, 1472, 1209, 1249, 735
• Hashmap/Hashset:
  • Leetcode 1, 146, 128, 73, 380, 49, 350, 299, 348
• Heap/Priority Queue:
  • Leetcode 973, 347, 23, 264, 1086, 88, 692, 378, 295, 767, 1438, 895
• Map:
  • Leetcode 729, 981, 846, 218, 480, 318

Binary Search:

• Three templates: [L, R), [L, R], Breakpoint.
• Explicit:
  • Leetcode 34, 33, 1095, 162, 278, 74, 240
• Implicit:
  • Leetcode 69, 540, 644, 528, 1300, 1060, 1062, 1891

Two Pointer:

• Basics: Same direction, opposite direction, or facing each other.
• Opposite (mostly palindrome):
  • Leetcode 409, 125, 5
• Facing (two sum based):
  • Leetcode 1, 167, 15, 16, 18, 454, 277, 11
• Same direction (sliding window):
  • Leetcode 283, 26, 395, 340, 424, 76, 3, 1004

DFS (Depth-First Search):

• Tree-based DFS:
  • Leetcode 543, 226, 101, 951, 124, 236, 105, 104, 987, 1485, 572, 863, 1110
• BST:
  • Leetcode 230, 98, 270, 235, 669, 700, 108, 333, 285
• Graph-based DFS:
  • Leetcode 341, 394, 51, 291, 126, 93, 22, 586, 301, 37, 212, 1087, 399, 1274, 1376, 694, 131
• Permutation/Combination DFS:
  • Leetcode 17, 39, 78, 46, 77, 698, 526
• Memoization Search:
  • Leetcode 139, 72, 377, 1235, 1335, 1216, 97, 472, 403, 329

Topological Sort:

• Leetcode 207, 444, 269, 310, 366

BFS (Breadth-First Search):

• Tree-based BFS:
  • Leetcode 102, 103, 297, 314
• Graph-based BFS:
  • Leetcode 200, 133, 127, 490, 323, 130, 752, 815, 1091, 542, 1293

Prefix Sum:

• Leetcode 53, 1423, 1031, 523, 304

Above are high-frequency topics. Below are medium-frequency topics.

Union Find:

• Leetcode 721, 547, 737, 305

Trie (Prefix Tree):

• Leetcode 208, 211, 1268, 212

Monotone Stack/Queue:

• Leetcode 85, 84, 907, 739, 901, 503, 239

Sweep Line:

• Leetcode 253, 218, 759

Dynamic Programming:

• Leetcode 674, 62, 70, 64, 368, 300, 354, 256, 121, 55, 45
• Leetcode 132, 312, 1143, 1062, 718, 174, 115, 72, 91, 639
• Leetcode 712, 221, 1277, 198, 213, 740, 87, 1140, 322, 518
• Leetcode 1048, 44, 10, 32, 1235, 1043, 926