DSA Interview Patterns
Six patterns that cover most array and string rounds in campus interviews
6 lessons · 1.2 hours of reading
· Intermediate · 3 free
Instead of a thousand random problems, learn the six patterns they are built from. Each lesson explains one pattern, then solves a full problem with it, from the brute force to the optimal code.
Lessons
1
Two pointers
Turn an O(n squared) nested loop into a single O(n) pass over a sorted array.
2
Sliding window
Reuse the previous window instead of recomputing it - contiguous subarray problems in O(n).
3
Prefix sums
Precompute once, answer any range-sum query in O(1) - and count subarrays with a given sum.
4
Hashing for frequency problems
Counting with a hash map, and the array-of-26 trick that is faster than a map.
5
Binary search on the answer
When the array is not sorted but the answer space is - the pattern most candidates never see.
6
Stacks and next greater element
The monotonic stack: every element pushed once and popped once, so O(n) instead of O(n squared).
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