Problem List

All LeetCode problems organized by pattern. Solve in priority order within each section.

Legend:

  • ✅ Must-do — foundational, appears in almost every interview loop
  • ⚡ High value — strong ROI, top-tier company favorite
  • 📖 Learning problem — builds intuition, less common in interviews

HashMap

# Problem Difficulty Frequency Priority
1 Two Sum Easy Very High ✅
49 Group Anagrams Medium Very High ✅
128 Longest Consecutive Sequence Medium Very High ✅
217 Contains Duplicate Easy High ✅
242 Valid Anagram Easy High ✅
347 Top K Frequent Elements Medium Very High ✅
560 Subarray Sum Equals K Medium High ✅
383 Ransom Note Easy Medium ⚡
290 Word Pattern Easy Medium ⚡
454 4Sum II Medium Medium ⚡
525 Contiguous Array Medium Medium ⚡
974 Subarray Sums Divisible by K Medium Medium ⚡

Two Pointers

# Problem Difficulty Frequency Priority
125 Valid Palindrome Easy High ✅
167 Two Sum II - Sorted Array Medium High ✅
15 3Sum Medium Very High ✅
11 Container With Most Water Medium Very High ✅
42 Trapping Rain Water Hard Very High ✅
18 4Sum Medium Medium ⚡
283 Move Zeroes Easy High ✅
344 Reverse String Easy Medium ⚡
977 Squares of a Sorted Array Easy Medium ⚡
16 3Sum Closest Medium Medium ⚡

Sliding Window

# Problem Difficulty Frequency Priority
643 Maximum Average Subarray I Easy Medium ✅
3 Longest Substring Without Repeating Characters Medium Very High ✅
567 Permutation in String Medium High ✅
76 Minimum Window Substring Hard Very High ✅
239 Sliding Window Maximum Hard High ✅
424 Longest Repeating Character Replacement Medium High ✅
209 Minimum Size Subarray Sum Medium High ⚡
438 Find All Anagrams in a String Medium High ⚡
1004 Max Consecutive Ones III Medium Medium ⚡
1343 Number of Sub-arrays of Size K and Avg >= Threshold Medium Low 📖

Stack

# Problem Difficulty Frequency Priority
20 Valid Parentheses Easy Very High ✅
155 Min Stack Medium Very High ✅
739 Daily Temperatures Medium Very High ✅
853 Car Fleet Medium Medium ⚡
84 Largest Rectangle in Histogram Hard High ✅
150 Evaluate Reverse Polish Notation Medium High ✅
22 Generate Parentheses Medium High ✅
496 Next Greater Element I Easy Medium ⚡
503 Next Greater Element II Medium Medium ⚡
85 Maximal Rectangle Hard Medium ⚡
901 Online Stock Span Medium Medium ⚡

Linked List

# Problem Difficulty Frequency Priority
206 Reverse Linked List Easy Very High ✅
21 Merge Two Sorted Lists Easy Very High ✅
141 Linked List Cycle Easy High ✅
142 Linked List Cycle II Medium High ✅
19 Remove Nth Node From End Medium High ✅
143 Reorder List Medium High ✅
23 Merge K Sorted Lists Hard Very High ✅
160 Intersection of Two Linked Lists Easy High ⚡
234 Palindrome Linked List Easy Medium ⚡
92 Reverse Linked List II Medium Medium ⚡
138 Copy List with Random Pointer Medium High ✅
25 Reverse Nodes in K-Group Hard Medium ⚡

Recursion / Divide & Conquer

# Problem Difficulty Frequency Priority
509 Fibonacci Number Easy Low 📖
50 Pow(x, n) Medium High ✅
215 Kth Largest Element in an Array Medium Very High ✅
148 Sort List Medium High ✅
23 Merge K Sorted Lists Hard Very High ✅
240 Search a 2D Matrix II Medium Medium ⚡
315 Count of Smaller Numbers After Self Hard Medium ⚡
4 Median of Two Sorted Arrays Hard High ⚡

Tree DFS

# Problem Difficulty Frequency Priority
104 Maximum Depth of Binary Tree Easy Very High ✅
226 Invert Binary Tree Easy Very High ✅
543 Diameter of Binary Tree Easy Very High ✅
110 Balanced Binary Tree Easy High ✅
100 Same Tree Easy High ✅
572 Subtree of Another Tree Easy High ✅
112 Path Sum Easy High ✅
113 Path Sum II Medium Medium ⚡
124 Binary Tree Maximum Path Sum Hard Very High ✅
236 Lowest Common Ancestor of Binary Tree Medium Very High ✅
105 Construct Tree from Preorder and Inorder Medium High ✅
437 Path Sum III Medium High ⚡
297 Serialize and Deserialize Binary Tree Hard High ⚡

Tree BFS

# Problem Difficulty Frequency Priority
102 Binary Tree Level Order Traversal Medium Very High ✅
199 Binary Tree Right Side View Medium Very High ✅
1161 Maximum Level Sum of Binary Tree Medium Medium ⚡
103 Binary Tree Zigzag Level Order Medium High ✅
116 Populating Next Right Pointers Medium Medium ⚡
111 Minimum Depth of Binary Tree Easy Medium ⚡
637 Average of Levels in Binary Tree Easy Medium ⚡
515 Find Largest Value in Each Tree Row Medium Low 📖

BST

# Problem Difficulty Frequency Priority
700 Search in a BST Easy Medium ✅
701 Insert into a BST Medium Medium ✅
450 Delete Node in a BST Medium Medium ⚡
98 Validate Binary Search Tree Medium Very High ✅
230 Kth Smallest Element in BST Medium High ✅
235 Lowest Common Ancestor of BST Medium High ✅
108 Convert Sorted Array to BST Easy Medium ⚡
173 BST Iterator Medium Medium ⚡
285 Inorder Successor in BST Medium Medium ⚡

Heap / Priority Queue

# Problem Difficulty Frequency Priority
703 Kth Largest Element in Stream Easy High ✅
1046 Last Stone Weight Easy Medium ⚡
973 K Closest Points to Origin Medium Very High ✅
215 Kth Largest Element in an Array Medium Very High ✅
621 Task Scheduler Medium High ✅
355 Design Twitter Medium Medium ⚡
295 Find Median from Data Stream Hard High ✅
23 Merge K Sorted Lists Hard Very High ✅
502 IPO Hard Low 📖
632 Smallest Range Covering Elements from K Lists Hard Low 📖

Graph BFS / DFS on Grids

# Problem Difficulty Frequency Priority
200 Number of Islands Medium Very High ✅
695 Max Area of Island Medium High ✅
994 Rotting Oranges Medium Very High ✅
286 Walls and Gates Medium High ✅
417 Pacific Atlantic Water Flow Medium High ✅
130 Surrounded Regions Medium Medium ⚡
1020 Number of Enclaves Medium Medium ⚡
1091 Shortest Path in Binary Matrix Medium Medium ⚡
827 Making a Large Island Hard Medium ⚡
542 01 Matrix Medium High ✅

Graph DFS

# Problem Difficulty Frequency Priority
133 Clone Graph Medium High ✅
207 Course Schedule Medium Very High ✅
323 Number of Connected Components Medium High ✅
261 Graph Valid Tree Medium High ✅
332 Reconstruct Itinerary Hard Medium ⚡
1376 Time Needed to Inform All Employees Medium Medium ⚡
399 Evaluate Division Medium Medium ⚡
802 Find Eventual Safe States Medium Medium ⚡

Greedy

# Problem Difficulty Frequency Priority
455 Assign Cookies Easy Medium ✅
1710 Maximum Units on a Truck Easy Medium ✅
55 Jump Game Medium Very High ✅
45 Jump Game II Medium High ✅
435 Non-overlapping Intervals Medium High ✅
452 Minimum Number of Arrows to Burst Balloons Medium Medium ⚡
56 Merge Intervals Medium Very High ✅
57 Insert Interval Medium High ✅
134 Gas Station Medium High ⚡
763 Partition Labels Medium Medium ⚡
846 Hand of Straights Medium Medium ⚡
# Problem Difficulty Frequency Priority
704 Binary Search Easy High ✅
374 Guess Number Higher or Lower Easy Medium ✅
35 Search Insert Position Easy High ✅
33 Search in Rotated Sorted Array Medium Very High ✅
81 Search in Rotated Sorted Array II Medium Medium ⚡
153 Find Minimum in Rotated Sorted Array Medium Very High ✅
74 Search a 2D Matrix Medium High ✅
875 Koko Eating Bananas Medium High ✅
1011 Capacity To Ship Packages Within D Days Medium High ✅
410 Split Array Largest Sum Hard Medium ⚡
4 Median of Two Sorted Arrays Hard High ⚡
162 Find Peak Element Medium High ⚡
34 Find First and Last Position of Element Medium High ✅

Backtracking

# Problem Difficulty Frequency Priority
78 Subsets Medium Very High ✅
90 Subsets II Medium Medium ⚡
77 Combinations Medium High ✅
46 Permutations Medium Very High ✅
47 Permutations II Medium Medium ⚡
39 Combination Sum Medium Very High ✅
40 Combination Sum II Medium Medium ⚡
216 Combination Sum III Medium Medium ⚡
131 Palindrome Partitioning Medium High ✅
17 Letter Combinations of a Phone Number Medium High ✅
79 Word Search Medium High ✅
51 N-Queens Hard High ✅
52 N-Queens II Hard Low 📖
37 Sudoku Solver Hard Medium ⚡

Dynamic Programming — 1D

# Problem Difficulty Frequency Priority
70 Climbing Stairs Easy Very High ✅
746 Min Cost Climbing Stairs Easy High ✅
198 House Robber Medium Very High ✅
213 House Robber II Medium High ✅
322 Coin Change Medium Very High ✅
518 Coin Change II Medium High ✅
139 Word Break Medium Very High ✅
300 Longest Increasing Subsequence Medium Very High ✅
416 Partition Equal Subset Sum Medium High ✅
91 Decode Ways Medium High ✅
338 Counting Bits Easy Medium ⚡
152 Maximum Product Subarray Medium Very High ✅
53 Maximum Subarray (Kadane’s) Medium Very High ✅
377 Combination Sum IV Medium Medium ⚡

Dynamic Programming — 2D

# Problem Difficulty Frequency Priority
62 Unique Paths Medium High ✅
63 Unique Paths II Medium Medium ⚡
64 Minimum Path Sum Medium High ✅
1143 Longest Common Subsequence Medium Very High ✅
72 Edit Distance Medium High ✅
115 Distinct Subsequences Hard Medium ⚡
312 Burst Balloons Hard Medium ⚡
10 Regular Expression Matching Hard Medium ⚡
97 Interleaving String Medium Medium ⚡
309 Best Time to Buy and Sell Stock with Cooldown Medium Medium ⚡
123 Best Time to Buy and Sell Stock III Hard High ⚡
188 Best Time to Buy and Sell Stock IV Hard Medium 📖

Trie

# Problem Difficulty Frequency Priority
208 Implement Trie Medium High ✅
211 Design Add and Search Words Medium High ✅
212 Word Search II Hard Medium ⚡
648 Replace Words Medium Low 📖
421 Maximum XOR of Two Numbers in an Array Medium Low 📖

Union Find

# Problem Difficulty Frequency Priority
547 Number of Provinces Medium High ✅
684 Redundant Connection Medium High ✅
128 Longest Consecutive Sequence Medium Very High ✅
721 Accounts Merge Medium High ✅
1584 Min Cost to Connect All Points Medium Medium ⚡
990 Satisfiability of Equality Equations Medium Medium ⚡
1319 Number of Operations to Make Network Connected Medium Medium ⚡
685 Redundant Connection II Hard Low 📖

Topological Sort

# Problem Difficulty Frequency Priority
207 Course Schedule Medium Very High ✅
210 Course Schedule II Medium Very High ✅
269 Alien Dictionary Hard High ✅
329 Longest Increasing Path in a Matrix Hard Medium ⚡
1203 Sort Items by Groups Respecting Dependencies Hard Low 📖
310 Minimum Height Trees Medium Medium ⚡
444 Sequence Reconstruction Medium Low 📖

Dijkstra’s Algorithm

# Problem Difficulty Frequency Priority
743 Network Delay Time Medium High ✅
1631 Path With Minimum Effort Medium High ✅
787 Cheapest Flights Within K Stops Medium High ✅
1514 Path With Maximum Probability Medium Medium ⚡
778 Swim in Rising Water Hard Medium ⚡
1368 Minimum Cost to Make at Least One Valid Path Hard Low 📖

Bellman-Ford

# Problem Difficulty Frequency Priority
787 Cheapest Flights Within K Stops Medium High ✅
743 Network Delay Time Medium High ⚡
1334 Find City With Smallest Number of Neighbors Medium Medium ⚡
# Problem Difficulty Frequency Priority
1091 Shortest Path in Binary Matrix Medium Medium ⚡
505 The Maze II Medium Low 📖
1263 Minimum Moves to Move a Box to Target Hard Low 📖

Segment Tree

# Problem Difficulty Frequency Priority
307 Range Sum Query - Mutable Medium High ✅
315 Count of Smaller Numbers After Self Hard Medium ⚡
493 Reverse Pairs Hard Low 📖
218 The Skyline Problem Hard Medium ⚡
308 Range Sum Query 2D - Mutable Hard Low 📖

Fenwick Tree / BIT

# Problem Difficulty Frequency Priority
307 Range Sum Query - Mutable Medium High ✅
315 Count of Smaller Numbers After Self Hard Medium ✅
493 Reverse Pairs Hard Low 📖
327 Count of Range Sum Hard Low 📖

AVL Trees / Balanced BST

# Problem Difficulty Frequency Priority
1382 Balance a Binary Search Tree Medium Low ⚡
315 Count of Smaller Numbers After Self Hard Medium ⚡
480 Sliding Window Median Hard Medium ⚡

Suffix Array

# Problem Difficulty Frequency Priority
1044 Longest Duplicate Substring Hard Low 📖
1062 Longest Repeating Substring Medium Low 📖
718 Maximum Length of Repeated Subarray Medium Medium ⚡

Miscellaneous — Must Know

These don’t fit neatly into one pattern but appear very frequently.

# Problem Pattern Difficulty Frequency Priority
121 Best Time to Buy and Sell Stock Array Easy Very High ✅
238 Product of Array Except Self Array Medium Very High ✅
271 Encode and Decode Strings String Medium High ✅
371 Sum of Two Integers Bit Manipulation Medium High ✅
191 Number of 1 Bits Bit Manipulation Easy High ✅
268 Missing Number Bit Manipulation Easy High ✅
146 LRU Cache Design Medium Very High ✅
380 Insert Delete GetRandom O(1) Design Medium High ✅
252 Meeting Rooms Intervals Easy High ✅
253 Meeting Rooms II Intervals Medium Very High ✅