How to Solve Longest Uploaded Prefix

This problem requires efficiently maintaining the longest prefix of consecutively uploaded videos in a stream. The key is to track which videos have been uploaded and compute the prefix dynamically. Using a boolean array or Hash Table for uploads combined with binary search or segment tracking ensures fast updates and retrievals.

Problem Statement

You are managing a stream of n videos numbered 1 to n. Each video can be uploaded in any order, and you must support queries to determine the longest continuous prefix of uploaded videos. Implement a system to handle these uploads and prefix queries efficiently.

Specifically, design the LUPrefix class where you can call upload(video) to mark a video as uploaded, and longest() to return the length of the longest prefix where all videos from 1 up to i have been uploaded. Consider the performance when handling a high volume of uploads and prefix queries.

Examples

Example 1

Input: See original problem statement.

Output: See original problem statement.

Input ["LUPrefix", "upload", "longest", "upload", "longest", "upload", "longest"] [[4], [3], [], [1], [], [2], []] Output [null, null, 0, null, 1, null, 3]

Explanation LUPrefix server = new LUPrefix(4); // Initialize a stream of 4 videos. server.upload(3); // Upload video 3. server.longest(); // Since video 1 has not been uploaded yet, there is no prefix. // So, we return 0. server.upload(1); // Upload video 1. server.longest(); // The prefix [1] is the longest uploaded prefix, so we return 1. server.upload(2); // Upload video 2. server.longest(); // The prefix [1,2,3] is the longest uploaded prefix, so we return 3.

Constraints

• 1 <= n <= 105
• 1 <= video <= n
• All values of video are distinct.
• At most 2 * 105 calls in total will be made to upload and longest.
• At least one call will be made to longest.

Solution Approach

Boolean Array Tracking

Maintain an array of size n+1 where array[i] is true if video i has been uploaded. Update the array on each upload, then iterate from the last known prefix to find the new longest uploaded prefix efficiently.

Binary Search Over Prefix

Use binary search on the valid answer space from the last confirmed prefix to n. Check if all videos up to mid are uploaded. This reduces repeated linear scans and fits the binary search over valid answer pattern.

Union-Find Optimization

Treat each uploaded video as a node in a union-find structure to link consecutive uploaded videos. Query the root of video 1 to determine the current longest prefix. This handles fragmented uploads efficiently and avoids scanning the array repeatedly.

Complexity Analysis

Time complexity ranges from O(1) per upload to O(log n) per longest query if using binary search or union-find path compression. Space complexity is O(n) for tracking uploaded videos.

What Interviewers Usually Probe

• Ask about handling out-of-order uploads efficiently.
• Probe whether prefix computation can avoid scanning all videos.
• Check if candidate recognizes binary search over answer space pattern.

Common Pitfalls or Variants

Common pitfalls

• Iterating from 1 to n for each query leads to TLE with large n.
• Forgetting to mark videos as uploaded correctly in the tracking structure.
• Assuming uploads are sequential and failing with out-of-order inputs.

Follow-up variants

• Query the number of missing videos in a range instead of the prefix.
• Support batch uploads and compute longest prefix after each batch.
• Return all longest prefixes after a series of uploads instead of single queries.

How GhostInterview Helps

• Provides step-by-step reasoning to maintain the uploaded prefix array or set efficiently.
• Highlights the binary search over valid answer space pattern specific to this prefix problem.
• Simulates edge cases with out-of-order uploads to verify union-find or array approaches.

Topic Pages

• Hash Table
• Binary Search
• Union Find
• Design
• Binary Indexed Tree
• Segment Tree
• Heap (Priority Queue)
• Ordered Set

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FAQ

What is the main pattern behind Longest Uploaded Prefix?

The primary pattern is binary search over the valid answer space, ensuring efficient prefix computation without scanning all videos.

Can uploads be handled out of order?

Yes, the solution must support uploads in any order, tracking each video independently to update the longest prefix.

What data structures are most useful for this problem?

Boolean arrays, Hash Tables, and Union-Find structures efficiently track uploaded videos and compute the longest prefix.

How does binary search optimize longest prefix queries?

Instead of scanning sequentially, binary search finds the highest index where all videos are uploaded, reducing query time.

What is a common mistake when implementing LUPrefix?

A frequent error is assuming uploads are sequential and failing to handle gaps, which breaks the longest prefix calculation.