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The reality: what DE coding rounds ask
FreeLesson 2 of 78
Before you open LeetCode, find out which kind of coding round you are walking into. Data engineering loops use two different kinds, and they need different preparation.
Many companies use one of these, some use both, and almost all add SQL. Ask the recruiter one direct question: "Will the coding round be algorithm problems, or data-processing tasks on records?" Then read the most recent interview reports for that company and role, because the format changes between teams and years.
What comes up in DE coding rounds, and how deep to go
| Topic | In DE loops | How deep | Pages |
|---|---|---|---|
| Hash maps and sets: counting, grouping, two-sum | Very often | Must be automatic | 46, 59-60 |
| Arrays and strings: two pointers, prefix sums, parsing | Very often | Must be automatic | 45, 64 |
| Sorting with custom keys, stability | Often | Know when and why | 36, 51 |
| Sliding windows (fixed and variable) | Often | Easy and medium | 47, 61 |
| Heaps: top-k, merge k sorted, running median | Often | Easy and medium | 55, 59 |
| Stacks, queues, intervals | Often | Easy and medium | 48, 65 |
| Binary search (including "on the answer") | Sometimes | Know the template | 50 |
| Trees and graphs: BFS, DFS, topological sort | Sometimes | Classics only | 53, 54 |
| Recursion and backtracking | Sometimes | Small inputs only | 52 |
| Dynamic programming | Rarely | Four classics | 56 |
| Linked lists | Rarely | Three classics | 49 |
These labels are a starting map built from candidate reports and the loops my mentees bring back, not a promise. Software engineering roles go deeper on graphs and dynamic programming than DE roles do.
What the interviewer actually scores
I Correctness on the edge cases you were not told about: empty input, duplicates, ties, None. I Communication: you said the approach before typing and talked while coding. I Complexity: you stated time and space without being asked, and knew a better option exists. I Code quality: names that read, small functions, no copy-paste. I Follow-ups: "what if it is 50 GB?" This is where DE rounds differ from SDE rounds.
What changes with experience
| Level | Same problems, but they expect |
|---|---|
| 0 to 2 years | A working solution, clear talk, and the Big-O stated |
| 2 to 5 years | The optimal approach first time, edge cases unprompted, production-style code |
| 5+ years | Trade-offs, scaling (memory, partitions, parallelism) and design judgement on every answer |
Practice before you move on
- Pick one question type from this page and write down the input and the output you would confirm before coding it.
- Time yourself on Q1 of Practice set 0 and note the step where you lost the most time.