Coding interview practice

Blind 75: the full list, with approaches and 1v1 practice

The Blind 75 is the best-known short list of coding-interview problems: 75 questions that cover the patterns interviews return to again and again. Here is every problem, grouped by topic, each with the idea behind it, its time and space complexity, a common pitfall, and a way to practise it against a real opponent.

75 problems
18 topics
19 easy · 49 medium · 7 hard

How to work through the Blind 75

Go topic by topic. Patterns build on each other: hashing before sliding windows, trees before graphs, recursion before dynamic programming.
Give each problem 20–30 minutes before reading the approach. Then write the solution yourself, without copying.
Come back a few days later and solve it again from scratch, faster.
Once a topic feels easy alone, practise it under pressure: a timed duel against someone at your level shows what you really know.

Arrays & Hashing

#	Problem	Difficulty	Pattern
1	Contains Duplicate	Easy	Hash set
2	Valid Anagram	Easy	Character counting
3	Two Sum	Easy	Hash map of complements
4	Group Anagrams	Medium	Canonical key
5	Top K Frequent Elements	Medium	Counting + ordering
6	Product of Array Except Self	Medium	Prefix and suffix products
7	Encode and Decode Strings	Medium	Length-prefix framing
8	Longest Consecutive Sequence	Medium	Hash set run starts

Two Pointers

#	Problem	Difficulty	Pattern
9	Valid Palindrome	Easy	Two pointers
10	Three Sum	Medium	Sort + two pointers
11	Container With Most Water	Medium	Two pointers

Sliding Window

#	Problem	Difficulty	Pattern
12	Best Time to Buy and Sell Stock	Easy	Running minimum
13	Longest Substring Without Repeating Characters	Medium	Sliding window
14	Longest Repeating Character Replacement	Medium	Sliding window with max count
15	Minimum Window Substring	Hard	Sliding window with need counts

Stack

#	Problem	Difficulty	Pattern
16	Valid Parentheses	Easy	Stack

Binary Search

#	Problem	Difficulty	Pattern
17	Find Minimum in Rotated Sorted Array	Medium	Binary search
18	Search in Rotated Sorted Array	Medium	Binary search on the sorted half

Linked List

#	Problem	Difficulty	Pattern
19	Reverse Linked List	Easy	Pointer reversal
20	Merge Two Sorted Lists	Easy	Two-pointer merge
21	Reorder List	Medium	Middle + reverse + weave
22	Remove Nth Node From End of List	Medium	Two pointers a fixed gap apart
23	Linked List Cycle	Easy	Fast and slow pointers
24	Merge K Sorted Lists	Hard	Min-heap or divide and conquer

Trees

#	Problem	Difficulty	Pattern
25	Invert Binary Tree	Easy	Tree recursion
26	Maximum Depth of Binary Tree	Easy	Tree recursion or BFS
27	Same Tree	Easy	Parallel recursion
28	Subtree of Another Tree	Easy	Tree matching
29	Lowest Common Ancestor of a BST	Medium	BST walk
30	Binary Tree Level Order Traversal	Medium	Breadth-first search
31	Validate Binary Search Tree	Medium	Bounds recursion
32	Kth Smallest Element in a BST	Medium	In-order traversal
33	Construct Binary Tree from Preorder and Inorder Traversal	Medium	Divide and conquer
34	Binary Tree Maximum Path Sum	Hard	Post-order DFS with a global best
35	Serialize and Deserialize Binary Tree	Hard	Level-order encoding

Heap / Priority Queue

#	Problem	Difficulty	Pattern
36	Find Median from Data Stream	Hard	Two heaps

Backtracking

#	Problem	Difficulty	Pattern
37	Combination Sum	Medium	Backtracking
38	Word Search	Medium	Grid backtracking

Tries

#	Problem	Difficulty	Pattern
39	Implement Trie	Medium	Prefix tree
40	Design Add and Search Words	Medium	Trie + wildcard DFS
41	Word Search II	Hard	Trie-guided backtracking

Graphs

#	Problem	Difficulty	Pattern
42	Number of Islands	Medium	Flood fill
43	Clone Graph	Medium	Graph traversal with a map
44	Pacific Atlantic Water Flow	Medium	Reverse multi-source search
45	Course Schedule	Medium	Topological sort
46	Graph Valid Tree	Medium	Union-find
47	Number of Connected Components	Medium	Union-find or DFS

Advanced Graphs

#	Problem	Difficulty	Pattern
48	Alien Dictionary	Hard	Topological sort with a min-heap

1-D Dynamic Programming

#	Problem	Difficulty	Pattern
49	Climbing Stairs	Easy	1-D dynamic programming
50	House Robber	Medium	1-D dynamic programming
51	House Robber II	Medium	DP on two ranges
52	Longest Palindromic Substring	Medium	Expand around centre
53	Palindromic Substrings	Medium	Expand around centre
54	Decode Ways	Medium	1-D dynamic programming
55	Coin Change	Medium	Unbounded knapsack DP
56	Maximum Product Subarray	Medium	Track max and min
57	Word Break	Medium	1-D dynamic programming
58	Longest Increasing Subsequence	Medium	Patience sorting

2-D Dynamic Programming

#	Problem	Difficulty	Pattern
59	Unique Paths	Medium	2-D dynamic programming
60	Longest Common Subsequence	Medium	2-D dynamic programming

Greedy

#	Problem	Difficulty	Pattern
61	Maximum Subarray	Medium	Kadane's algorithm
62	Jump Game	Medium	Greedy reach

Intervals

#	Problem	Difficulty	Pattern
63	Insert Interval	Medium	Linear merge
64	Merge Intervals	Medium	Sort + sweep
65	Non-overlapping Intervals	Medium	Greedy by end
66	Meeting Rooms	Easy	Sort + adjacent check
67	Meeting Rooms II	Medium	Sweep line

Math & Geometry

#	Problem	Difficulty	Pattern
68	Rotate Image	Medium	Transpose + reverse
69	Spiral Matrix	Medium	Shrinking boundaries
70	Set Matrix Zeroes	Medium	In-place markers

Bit Manipulation

#	Problem	Difficulty	Pattern
71	Number of 1 Bits	Easy	Bit tricks
72	Counting Bits	Easy	DP on bits
73	Reverse Bits	Easy	Bit shifting
74	Missing Number	Easy	XOR or sum
75	Sum of Two Integers	Medium	Bitwise addition

Frequently asked questions

What is the Blind 75?

A list of 75 coding-interview problems first shared on the Blind forum by an engineer who wanted the smallest set that still covers the important patterns. It has become the most common starting list for interview preparation.

How long does the Blind 75 take?

At a few problems a day it usually takes several weeks, plus time to revisit the ones that did not stick. Speed comes from solving the same patterns again, not from adding more problems.

Blind 75 or NeetCode 150?

The NeetCode 150 is the Blind 75 plus 75 more problems on the same patterns. Start with the Blind 75; move on to a larger list once you can solve these reliably.

Which language should I use?

The one you know best. Interviewers care about the approach and clean code. DuelCoder supports TypeScript, JavaScript, Python and C++.

Are these the original LeetCode statements?

No. DuelCoder's statements, examples and tests are written independently; the problem names match the public Blind 75 list so you can find them. DuelCoder is not affiliated with LeetCode.