2424. Longest Uploaded Prefix

Problem

You are given a stream of n videos, each represented by a distinct number from 1 to n that you need to "upload" to a server. You need to implement a data structure that calculates the length of the longest uploaded prefix at various points in the upload process.

We consider i to be an uploaded prefix if all videos in the range 1 to i (inclusive) have been uploaded to the server. The longest uploaded prefix is the **maximum **value of i that satisfies this definition.

Implement the LUPrefix class:

• LUPrefix(int n) Initializes the object for a stream of n videos.

• void upload(int video) Uploads video to the server.

• int longest() Returns the length of the longest uploaded prefix defined above.

  Example 1:

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 <= 10^5

• 1 <= video <= 10^5

• All values of video are distinct.

• At most 2 * 10^5 calls in total will be made to upload and longest.

• At least one call will be made to longest.

Solution (Java)

class LUPrefix {
    Integer[] prefix;
    int index = 0;
    public LUPrefix(int n) {
        prefix = new Integer[n];
    }

    public void upload(int video) {
        prefix[video - 1] = 1;
        while(index < prefix.length && prefix[index] != null){
            index++;
        }
    }

    public int longest() {
        return index;
    }
}

/**
 * Your LUPrefix object will be instantiated and called as such:
 * LUPrefix obj = new LUPrefix(n);
 * obj.upload(video);
 * int param_2 = obj.longest();
 */

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).