Coding Fundamentals and Complexity for DS — Tech Interview Concept

What it is This is the practical toolkit of data structures, algorithms, and Big-O analysis you use to reason about runtime and memory. It helps you choose implementations that stay fast as data grows and to spot when code will blow up at scale.
Why interviewers ask about it Teams shipping ranking, ads, or integrity systems care whether your solution still works on tens of millions of rows. They’re probing if you can turn a correct idea into something that’s efficient in Python and common ML/data libraries, and if you can explain performance trade-offs clearly.
Core ideas to know

Big-O captures how runtime/space grow with input; know O(1), O(log n), O(n), O(n log n), O(n^2).
Python lists: append amortized O(1); insert/delete in middle O(n); membership tests O(n).
dict/set: average O(1) inserts/lookups; worst-case O(n) with poor hashing; ideal for membership, dedup, joins.
Sorting: built-ins are stable; typical O(n log n). Supplying a key function can reduce comparison overhead.
Heaps solve streaming top-k: maintain a size-k heap, overall O(n log k).
Prefer vectorized NumPy/pandas and library ops; avoid Python loops that add large constant factors.

A common pitfall Candidates recite Big-O but ignore real Python and data stack behavior. They write nested Python loops for pairwise operations (O(n^2)) that crawl on 10M rows instead of using vectorized operations, groupby/merge, or a heap-based top-k. Others use list membership in joins instead of sets/dicts, or sort full datasets when a heap or nsmallest would do. Interviewers notice when you can replace slow constructs with the right data structure and explain the resulting complexity.
Further reading

MIT OCW: Big-Oh and Big-Theta — clean, formal intro with examples you can adapt in interviews. (ocw.mit.edu)
Python Wiki: TimeComplexity — authoritative cheat sheet for list, dict, and set operations in CPython. (wiki.python.org)
scikit-learn: Computational Performance — how estimator training/prediction scale and practical tips for avoiding bottlenecks. (scikit-learn.org)

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