How do you solve Longest Uploaded Prefix on LeetCode?

Longest Uploaded Prefix (LeetCode #2424) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(n) space complexity. Key insight: Tracking the longest prefix efficiently can save time and avoid unnecessary checks.

What is the brute force solution for Longest Uploaded Prefix?

The brute force approach for Longest Uploaded Prefix is Brute Force, with O(n²) time complexity and O(n) space complexity. The brute force approach checks each video upload one by one to determine the longest prefix. It does this by iterating through the uploaded videos and checking if all videos from 1 to i are uploaded. The optimal Optimal Solution approach improves this to O(n).

What are the interview tips for Longest Uploaded Prefix?

Explain your thought process clearly while coding, especially when transitioning from brute force to optimal solutions. Consider edge cases, such as uploading videos in a non-sequential order. Practice similar problems to become familiar with patterns like maintaining state with arrays or hash maps.

What are common mistakes when solving Longest Uploaded Prefix?

Not maintaining the longest prefix pointer correctly, leading to incorrect results. Forgetting to check the bounds of the array when accessing uploaded videos.

#2424

Longest Uploaded Prefix

Medium

Hash TableBinary SearchUnion-FindDesignBinary Indexed TreeSegment TreeHeap (Priority Queue)Ordered SetHash MapArray

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(n)	O(n)

💡

Intuition

Time O(n)Space O(n)

The optimal solution uses an array to track uploaded videos and a pointer to track the longest prefix. This allows us to efficiently check the longest prefix after each upload without re-checking all previous uploads.

⚙️

Algorithm

3 steps

1Step 1: Initialize an array to keep track of uploaded videos and a pointer for the longest prefix.
2Step 2: When uploading a video, mark it as uploaded.
3Step 3: Increment the pointer while the next video in sequence is uploaded.

solution.py12 lines

1class LUPrefix:
2    def __init__(self, n: int):
3        self.uploaded = [False] * (n + 1)
4        self.longest_prefix = 0
5
6    def upload(self, video: int) -> None:
7        self.uploaded[video] = True
8        while self.longest_prefix + 1 < len(self.uploaded) and self.uploaded[self.longest_prefix + 1]:
9            self.longest_prefix += 1
10
11    def longest(self) -> int:
12        return self.longest_prefix

ℹ

Complexity note: The time complexity is O(n) because each upload can potentially increment the longest prefix pointer only once, and we only traverse the uploaded array once per upload.

1Tracking the longest prefix efficiently can save time and avoid unnecessary checks.
2Using an array to mark uploaded videos allows constant time checks for the longest prefix.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.