LeetCode in Kotlin

2424. Longest Uploaded Prefix

Medium

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 <= 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

class LUPrefix(n: Int) {
    private val res: BooleanArray
    private var count = 0

    init {
        res = BooleanArray(n + 1)
    }

    fun upload(video: Int) {
        res[video - 1] = true
    }

    fun longest(): Int {
        while (res[count]) {
            count++
        }
        return count
    }
}

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