Pattern 15: Blind 75 — The Essential Interview Problem Set

The Blind 75 is a curated list of 75 LeetCode problems created by Yangshun Tay in 2018. It covers the most frequently asked coding interview questions across 11 categories. Mastering these problems gives you the patterns and techniques needed to solve the vast majority of coding interview questions at companies like Google, Meta, Amazon, Apple, and Microsoft.

Philosophy: Don’t memorize solutions. Learn the patterns and techniques. Once you recognize a pattern, the solution approach becomes clear.

Categories & Problem Counts

#	Category	Count	Key Patterns
1	Array	10	HashMap, Two Pointers, Binary Search, Kadane’s, Prefix Products
2	Binary / Bit Manipulation	5	XOR, Bit Shifting, Brian Kernighan’s trick
3	Dynamic Programming	11	1D DP, 2D DP, State Machines, Knapsack
4	Graph	8	DFS, BFS, Topological Sort, Union Find
5	Interval	4	Sort by start, Greedy merge
6	Linked List	6	Two Pointers, Fast/Slow, Reversal
7	Matrix	4	Layer traversal, Transpose, DFS Backtracking
8	String	10	Sliding Window, HashMap, Stack, Expand Around Center
9	Tree	11	DFS, BFS, Inorder, Preorder, Recursion
10	Trie	3	Prefix Tree, DFS + Trie
11	Heap	3	Min/Max Heap, Two Heaps, Bucket Sort
	Total	75

Array (10 problems)

#	Problem	LC #	Difficulty	Key Pattern
1	Two Sum	1	Easy	HashMap complement lookup
2	Best Time to Buy and Sell Stock	121	Easy	Track min price, greedy
3	Contains Duplicate	217	Easy	HashSet
4	Product of Array Except Self	238	Medium	Prefix/suffix products
5	Maximum Subarray	53	Medium	Kadane’s algorithm
6	Maximum Product Subarray	152	Medium	Track min and max product
7	Find Minimum in Rotated Sorted Array	153	Medium	Binary search
8	Search in Rotated Sorted Array	33	Medium	Modified binary search
9	3Sum	15	Medium	Sort + two pointers
10	Container With Most Water	11	Medium	Two pointers from edges

Binary / Bit Manipulation (5 problems)

#	Problem	LC #	Difficulty	Key Pattern
1	Sum of Two Integers	371	Medium	XOR + carry with AND shift
2	Number of 1 Bits	191	Easy	n & (n-1) trick
3	Counting Bits	338	Easy	DP: ans[i] = ans[i>>1] + (i&1)
4	Missing Number	268	Easy	XOR all indices and values
5	Reverse Bits	190	Easy	Bit-by-bit shift

Dynamic Programming (11 problems)

#	Problem	LC #	Difficulty	Key Pattern
1	Climbing Stairs	70	Easy	Fibonacci-style DP
2	Coin Change	322	Medium	Unbounded knapsack DP
3	Longest Increasing Subsequence	300	Medium	1D DP or patience sort
4	Longest Common Subsequence	1143	Medium	2D DP
5	Word Break	139	Medium	DP + HashSet
6	Combination Sum IV	377	Medium	1D DP permutation count
7	House Robber	198	Medium	Skip-adjacent DP
8	House Robber II	213	Medium	Circular array DP
9	Decode Ways	91	Medium	1D DP with 1/2 digit checks
10	Unique Paths	62	Medium	2D grid DP
11	Jump Game	55	Medium	Greedy reachability

Graph (8 problems)

#	Problem	LC #	Difficulty	Key Pattern
1	Clone Graph	133	Medium	BFS/DFS + HashMap clone
2	Course Schedule	207	Medium	Topological sort / cycle detection
3	Pacific Atlantic Water Flow	417	Medium	Multi-source reverse DFS
4	Number of Islands	200	Medium	DFS/BFS flood fill
5	Longest Consecutive Sequence	128	Medium	HashSet streak heads
6	Alien Dictionary	269	Hard	Topological sort from word order
7	Graph Valid Tree	261	Medium	Union Find + edge count
8	Number of Connected Components	323	Medium	Union Find / DFS

Interval (4 problems)

#	Problem	LC #	Difficulty	Key Pattern
1	Insert Interval	57	Medium	Find overlap window, merge
2	Merge Intervals	56	Medium	Sort by start + merge
3	Non-overlapping Intervals	435	Medium	Greedy: keep earliest end
4	Meeting Rooms II	253	Medium	Min-heap or sweep line

Linked List (6 problems)

#	Problem	LC #	Difficulty	Key Pattern
1	Reverse Linked List	206	Easy	Iterative pointer swap
2	Linked List Cycle	141	Easy	Floyd’s slow/fast pointers
3	Merge Two Sorted Lists	21	Easy	Dummy head + merge
4	Merge K Sorted Lists	23	Hard	Min-heap or divide & conquer
5	Remove Nth Node From End	19	Medium	Two pointers with gap
6	Reorder List	143	Medium	Find mid + reverse + zip merge

Matrix (4 problems)

#	Problem	LC #	Difficulty	Key Pattern
1	Set Matrix Zeroes	73	Medium	First row/col as markers
2	Spiral Matrix	54	Medium	Layer-by-layer shrink
3	Rotate Image	48	Medium	Transpose + reverse rows
4	Word Search	79	Medium	DFS backtracking

String (10 problems)

#	Problem	LC #	Difficulty	Key Pattern
1	Longest Substring Without Repeating	3	Medium	Sliding window + HashSet
2	Longest Repeating Character Replacement	424	Medium	Sliding window + frequency
3	Minimum Window Substring	76	Hard	Sliding window + frequency map
4	Valid Anagram	242	Easy	Frequency count
5	Group Anagrams	49	Medium	Sorted key + HashMap
6	Valid Parentheses	20	Easy	Stack matching
7	Longest Palindromic Substring	5	Medium	Expand around center
8	Palindromic Substrings	647	Medium	Expand around center
9	Encode and Decode Strings	271	Medium	Length-delimited encoding
10	Valid Palindrome	125	Easy	Two pointers + skip

Tree (11 problems)

#	Problem	LC #	Difficulty	Key Pattern
1	Maximum Depth of Binary Tree	104	Easy	DFS recursion
2	Same Tree	100	Easy	DFS simultaneous traversal
3	Invert Binary Tree	226	Easy	DFS swap children
4	Binary Tree Maximum Path Sum	124	Hard	DFS + global max tracking
5	Binary Tree Level Order Traversal	102	Medium	BFS with queue
6	Serialize and Deserialize Binary Tree	297	Hard	Preorder + delimiter
7	Subtree of Another Tree	572	Easy	DFS + subtree match
8	Construct from Preorder and Inorder	105	Medium	Recursion + index map
9	Validate Binary Search Tree	98	Medium	Inorder or range check
10	Kth Smallest Element in BST	230	Medium	Inorder traversal
11	Lowest Common Ancestor of BST	235	Medium	BST property split point

Trie (3 problems)

#	Problem	LC #	Difficulty	Key Pattern
1	Implement Trie (Prefix Tree)	208	Medium	TrieNode array of children
2	Add and Search Word	211	Medium	Trie + DFS for wildcards
3	Word Search II	212	Hard	Trie + board DFS

Heap (3 problems)

#	Problem	LC #	Difficulty	Key Pattern
1	Merge K Sorted Lists	23	Hard	Min-heap
2	Top K Frequent Elements	347	Medium	Bucket sort or min-heap
3	Find Median from Data Stream	295	Hard	Two heaps (max + min)

Study Strategy

Recommended Order

Array — Foundation; teaches HashMap, two pointers, binary search
String — Reinforces sliding window and HashMap skills
Linked List — Pointer manipulation mastery
Tree — Recursion and DFS/BFS foundations
Graph — Extends tree traversal to general graphs
Binary — Quick wins with bit tricks
Matrix — 2D traversal patterns
Interval — Sort-and-merge strategy
Heap — Priority queue applications
Trie — Advanced string matching
Dynamic Programming — Save for last; requires all prior patterns

Time Targets

Difficulty	Target Time	Practice Time
Easy	10-15 min	20-30 min
Medium	20-30 min	40-60 min
Hard	30-45 min	60-90 min

The Pattern Recognition Loop

Read the problem — identify keywords
Match to a known pattern (this guide helps)
Write pseudocode BEFORE coding
Code the brute force first
Optimize to the pattern-based solution
Dry run with a sample input
Analyze time and space complexity

Cross-Reference with Existing Pattern Directories

Many Blind 75 problems are also covered in your numbered pattern directories:

Blind 75 Problem	Also In Directory
LC 121 Best Time to Buy and Sell Stock	`00-Foundations/`
LC 238 Product of Array Except Self	`00-Foundations/`
LC 125 Valid Palindrome	`01-Two-Pointers/`
LC 217 Contains Duplicate	`03-Hashing-Frequency-Maps/`
LC 242 Valid Anagram	`03-Hashing-Frequency-Maps/`
LC 57 Insert Interval	`05-Merge-Intervals/`
LC 253 Meeting Rooms II	`05-Merge-Intervals/`
LC 21 Merge Two Sorted Lists	`07-Linked-List-Reversal/`
LC 226 Invert Binary Tree	`09-DFS/`
LC 100 Same Tree	`09-DFS/`
LC 39 Combination Sum	`11-Backtracking-Subsets/`
LC 46 Permutations	`11-Backtracking-Subsets/`
LC 704 Binary Search	`12-Binary-Search/`
LC 70 Climbing Stairs	`14-Dynamic-Programming/`
LC 322 Coin Change	`14-Dynamic-Programming/`
LC 62 Unique Paths	`14-Dynamic-Programming/`
LC 300 LIS	`14-Dynamic-Programming/`