Introduction to DSA

A data structure is a way of organizing data (an array, a list, a tree) so certain operations are fast. An algorithm is a step-by-step procedure for solving a problem. Choosing the right structure often makes an algorithm dramatically faster.

Big-O notation describes how an algorithm's running time grows as input size grows — O(n) means roughly proportional to input size; O(n²) means it grows much faster (e.g. a nested loop over the same list).

This subject shows up constantly in technical interviews not because you'll hand-write sorting algorithms at work, but because it demonstrates how you reason about efficiency and tradeoffs.