1. Arrays and Strings
Arrays and strings form the foundation of almost every coding interview. Practice problems involving two-pointer techniques, sliding windows and in-place manipulation, since these patterns reappear constantly across different questions.
2. Linked Lists
Linked lists test your understanding of pointers and memory references. Focus especially on reversing a linked list, detecting cycles, and merging two sorted lists — these three problem types cover the majority of linked-list interview questions.
3. Stacks and Queues
Stacks and queues are essential for problems involving order of operations, such as balanced parentheses checking, and for implementing breadth-first traversal in graphs. Understanding when to use a stack versus a queue is often the real test, more than implementation itself.
4. Trees and Binary Search Trees
Tree traversal — inorder, preorder, postorder and level-order — comes up repeatedly, along with problems on validating binary search trees and finding the lowest common ancestor. Drawing the tree out on paper while solving a problem dramatically reduces mistakes.
5. Hash Maps
Hash maps are frequently the key to converting a slow, brute-force solution into a fast one. Many "find the pair" or "count frequency" style problems become trivial once you recognise that a hash map can look up values in constant time.
How to Actually Practice These
Rather than jumping between random problems, pick one data structure at a time and solve 8–10 problems focused specifically on it before moving to the next. This builds pattern recognition far faster than solving problems in a random, mixed order.
Beyond Memorising Code
Interviewers care more about whether you can explain your reasoning and identify the right data structure for a new, unseen problem than whether you have memorised a specific solution. Practice explaining your thought process out loud as you solve problems, not just writing the code silently.
