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Arrays & Strings
most common · master first
The highest-frequency category at top-tier companies.
Two pointers, sliding window, and hashing
cover most of these. If you nail this category, you handle ~30% of coding rounds. Focus on writing the patterns cleanly and fast.
15 problems
expand all
01
Two Sum
EASY
HashMap
LC 1
▶
02
Best Time to Buy & Sell Stock
EASY
Greedy / Min-tracking
LC 121
▶
03
Maximum Subarray (Kadane)
MEDIUM
DP / Kadane
LC 53
▶
04
Product of Array Except Self
MEDIUM
Prefix/Suffix
LC 238
▶
05
Longest Substring Without Repeating Characters
MEDIUM
Sliding Window
LC 3
▶
06
Minimum Window Substring
HARD
Sliding Window
LC 76
▶
07
Trapping Rain Water
HARD
Two Pointers
LC 42
▶
08
Container With Most Water
MEDIUM
Two Pointers
LC 11
▶
09
Merge Intervals
MEDIUM
Sorting
LC 56
▶
10
Search in Rotated Sorted Array
MEDIUM
Binary Search
LC 33
▶
11
Longest Consecutive Sequence
MEDIUM
HashSet
LC 128
▶
12
Valid Anagram / Group Anagrams
MEDIUM
Hashing
LC 242 / 49
▶
13
Next Permutation
MEDIUM
Array Manipulation
LC 31
▶
14
Rotate Image (90°)
MEDIUM
Matrix
LC 48
▶
15
Spiral Matrix
MEDIUM
Matrix
LC 54
▶
Next pattern
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