Same-Direction: Read and Write Pointers

Concepts covered: pyReadWritePointers, pyDeduplication

The second two-pointer sub-pattern has both pointers starting at the same end and moving in the same direction. One pointer reads through the input. The other pointer marks the write position in the output. The read pointer moves every step. The write pointer only moves when a valid element is found. This is the pattern for in-place array modifications: removing duplicates, removing a specific value, moving zeroes to the end. Remove Duplicates from Sorted Array Given a sorted array, remove duplicates in-place and return the new length. This is LeetCode 26, and it is asked at every level of data engineering interviews because it maps directly to deduplication in pipelines. The read pointer scans forward. The write pointer only advances when a new unique value is found. The invariant: everyt

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This section is part of the Two Pointers: Beginner lesson on DataDriven, a free data engineering interview prep platform. Each section includes explanations, worked examples, and hands-on code challenges that execute in real time. SQL queries run against a live database. Python runs in a sandboxed Docker container. Data modeling problems validate against interactive schema canvases. All content is framed around what data engineering interviewers actually test at companies like Meta, Google, Amazon, Netflix, Stripe, and Databricks.

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