Amazon·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Amazon SWE interview with a stack-based algorithm problem. The question was straightforward to understand but the O(N^2) constraint meant you had to actually think before coding.

Questions Asked (1)

Given an array of integers representing task priorities that are processed right to left, return an array where each element is the index distance to the first smaller value to its right (or 0 if none exists). You must do better than O(N^2).

Algorithms & Data Structures

Author's notes

Knew immediately this was a monotonic stack problem but explaining WHY took me longer than it should have.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Use a monotonic stack to efficiently find the next smaller element to the right for each index. Traverse the array from right to left, maintaining a stack of indices with increasing values, and for each element, pop indices until the top has a smaller value, then compute the distance. This achieves O(N) time and O(N) space.

Pro tip: Clarify the definition of 'first smaller value to its right' and confirm whether the distance is measured in indices or steps; also mention that the stack stores indices, not values, to easily compute distances.

1. Understand the problem and constraints

Restate the problem: for each element, find the index distance to the nearest smaller element to its right, or 0 if none. Note the requirement to do better than O(N^2).

2. Choose the right data structure

Select a monotonic stack to keep track of potential next smaller elements. The stack will store indices and maintain increasing values from top to bottom.

3. Traverse from right to left

Iterate through the array starting from the last index. For each element, pop from the stack while the top element's value is greater than or equal to the current element.

4. Compute distances and update stack

If the stack is not empty, the top index is the next smaller element; compute the distance as top - current index. If empty, distance is 0. Then push the current index onto the stack.

5. Analyze complexity and edge cases

Explain that each element is pushed and popped at most once, giving O(N) time and O(N) space. Discuss edge cases like all increasing, all decreasing, or duplicate values.

Key Points to Mention

Monotonic stack technique for next smaller element problems
Time and space complexity analysis: O(N) time, O(N) space
Handling duplicates: use >= when popping to ensure strict smaller
Edge cases: empty array, single element, strictly increasing/decreasing arrays
Comparison with brute force O(N^2) approach and why stack is better
Storing indices instead of values to compute distances easily

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.