AmazonOnline Assessment

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12 Sets2 Questions/Set90 minSliding Window & Monotonic DequeMin-Heap & Priority Queue OptimizationsGraph BFS & Disjoint Set Union (DSU)Greedy Interval & Partitioning AlgorithmsPrefix Sums & Boundary Array Lookups

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Practice Sets

Community OA — SDE 1 / SDE 2 · 0-1 YOE · 2026

2 questions

Q1.

AWS Server Scaling (Minimum Operations to Non-Decreasing)

ArraysGreedyAlgorithms

Medium

Problem Statement

AWS provides scalable systems. A set of n servers are used for horizontally scaling an application. The goal is to have the computational power of the servers in non-decreasing order.

To do so, you can increase the computational power of each server in any contiguous segment by x (x > 0). Choose the values of x such that after all operations, the computational powers are in non-decreasing order, and the sum of the x values is minimum.

Given the initial array power[n] representing the computational powers of the servers, determine the minimum sum of x values needed to make the array non-decreasing.

Input Format

The first line contains an integer n denoting the number of servers.
The next n lines contain integers denoting the initial computational power of each server (power[i]).

Output Format

Return a long integer representing the minimum sum of x values.

Constraints

1 <= n <= 10^5
1 <= power[i] <= 10^9

Sample Test Cases

Example 1

Input

Output

Explanation

Initial array: [3, 4, 1, 6, 2].

- Add 3 units to subarray (2, 4): [3, 4, 4, 9, 5].

- Add 4 units to subarray (4, 4): [3, 4, 4, 9, 9].

Array is now in non-decreasing order. Sum of x = 3 + 4 = 7.

Example 2

Input

Output

Explanation

Initial array: [3, 2, 1].

- Drop from index 0 to 1 is 3 - 2 = 1.

- Drop from index 1 to 2 is 2 - 1 = 1.

Total minimum sum of x = 1 + 1 = 2.

Approach:1. Let difference array D[i] = power[i] - power[i-1] for i = 1..n-1. The array is non-decreasing iff D[i] >= 0 for all i. 2. When adding x to subarray [L, R], D[L] increases by x and D[R+1] decreases by x. To avoid penalizing later elements, optimal operations always extend to the end (R = n-1, suffix operations). 3. Any decrease where power[i] < power[i-1] represents an unavoidable deficit of (power[i-1] - power[i]) that must be compensated by adding at least that amount to a suffix starting at index i. 4. Therefore, minimum total sum of x is simply the sum of all downward drops: ans = sum(max(0, power[i-1] - power[i])) for all i from 1 to n-1. 5. Operates in O(N) time and O(1) space.

Q2.

Workflow Commenting System (Spring Boot Backend Bugfix)

Spring BootREST APIBackend DebuggingJava

Hard

Problem Statement

Workflow is an app that allows teams to manage and track their work efficiently. The commenting system enables team members to discuss issues, provide updates, and collaborate on solutions. Currently, this feature is not functioning correctly, affecting team communication and collaboration.

Issue Summary:

When users attempt to add comments to issues or edit their own comments, the changes do not appear or persist. Fix the comment creation and editing feature in the backend.

Requirements & HTTP Error Handling:

1. POST /api/issues/{issueId}/comments:

- Authentication: If user is not authenticated, return 401 Unauthorized with {"message": "Not authenticated"}.

- Validation: If content is empty, null, or whitespace only, return 400 Bad Request with {"message": "Content is required"}.

- Persistence: Associate comment with the issue and authenticated user. Return 201 Created with the saved comment.

2. PUT /api/comments/{commentId}:

- Authentication: If user is not authenticated, return 401 Unauthorized with {"message": "Not authenticated"}.

- Validation: If content is empty or missing, return 400 Bad Request with {"message": "Content is required"}.

- Existence: If comment does not exist, return 404 Not Found with {"message": "Comment not found"}.

- Authorization: If authenticated user is NOT the creator/owner of the comment, return 403 Forbidden with {"message": "Not authorized"}.

- Persistence: Update content, set isEdited = true, update updatedAt timestamp, save and return 200 OK.

Input Format

REST API HTTP Request Payloads:
- POST /api/issues/{issueId}/comments with JSON body: {"content": "..."}
- PUT /api/comments/{commentId} with JSON body: {"content": "..."}

Output Format

JSON HTTP Responses:
- 201 Created / 200 OK with comment details
- 400 Bad Request: {"message": "Content is required"}
- 401 Unauthorized: {"message": "Not authenticated"}
- 403 Forbidden: {"message": "Not authorized"}
- 404 Not Found: {"message": "Comment not found"}

Constraints

Spring Boot 3.x / Java 17+
JPA / Hibernate entity relationships with User and Issue entities.

Sample Test Cases

Example 1

Input

POST /api/issues/1/comments
Headers: Authorization: Bearer <valid_token>
Body: {"content": "Initial update on investigation"}

Output

HTTP 201 Created
{
  "id": 101,
  "content": "Initial update on investigation",
  "isEdited": false,
  "user": {"email": "[email protected]"}
}

Explanation

Authenticated user creates comment with valid content. Comment is persisted and returned with 201 status.

Example 2

Input

PUT /api/comments/101
Headers: Authorization: Bearer <other_user_token>
Body: {"content": "Unauthorized edit attempt"}

Output

HTTP 403 Forbidden
{"message": "Not authorized"}

Explanation

User attempting edit is not comment owner. Access is forbidden.

Approach:1. In CommentService, implement addComment(Long issueId, CommentRequest request, User currentUser): - Check currentUser != null, throw UnauthorizedException('Not authenticated') if missing. - Check request.getContent() != null && !request.getContent().trim().isEmpty(), throw BadRequestException('Content is required'). - Fetch issue by issueId or throw NotFoundException('Issue not found'). - Build Comment entity with issue, user, isEdited=false, save via commentRepository. 2. In CommentService, implement editComment(Long commentId, CommentRequest request, User currentUser): - Check currentUser != null, throw UnauthorizedException('Not authenticated'). - Check request.getContent() != null && !request.getContent().trim().isEmpty(), throw BadRequestException('Content is required'). - Fetch comment by commentId or throw NotFoundException('Comment not found'). - Verify comment.getUser().getId().equals(currentUser.getId()), throw ForbiddenException('Not authorized') if mismatch. - Update content, isEdited=true, updatedAt=Instant.now(), save and return comment.

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AmazonOnline Assessment

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Practice Sets

Community OA — SDE 1 / SDE 2 · 0-1 YOE · 2026

AWS Server Scaling (Minimum Operations to Non-Decreasing)

Problem Statement

Input Format

Output Format

Constraints

Sample Test Cases

Workflow Commenting System (Spring Boot Backend Bugfix)

Problem Statement

Input Format

Output Format

Constraints

Sample Test Cases

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