Amazon OA Coding Questions: Patterns That Repeat

Pair sums, anagram detection, frequency of characters, finding duplicates — these appear in nearly every OA. The signature move is a single pass with a hash map plus an O(1) lookup.

Practice variants like counting pairs with a target difference and longest substring without repeating characters — Amazon reuses these families.

Subarray problems — maximum sum, target sum subarrays, window with constraint — usually reduce to a sliding window or prefix sum map. The clue is a contiguous subarray or substring constraint.

These problems punish naive double loops under big constraints, so always compute the window math before coding.

Interval problems, scheduling, and selection problems hide behind a sort: sort by end time, then scan greedily. Merge intervals, meeting rooms, and similar variants are frequent OA material.

The complexity is usually O(n log n) from the sort — state it before writing code so you know you are done.

Grid traversal, connected components, and shortest path with a twist show up regularly. Practice BFS for shortest steps and DFS for counting or grouping, with visited arrays to avoid cycles.

Edge cases: blocked cells, multi-source starts, and large grids that need iterative (not recursive) DFS.

Simple recursion-plus-cache problems — climbing stairs, house robber, unique paths — appear often at easy-medium level. Recognize the state (position, remaining, carry) and write the memo.

If a problem has an optimal substructure and asks for count, max, or min, DP or greedy is likely the route.