Dynamic Array Problem (Hacker rank)

Question

I am trying to solve the Dynamic Array problem on HackerRank:

• Create a list, seqList, of N empty sequences, where each sequence is indexed from 0 to N-1. The elements within each of the N sequences also use 0-indexing.
• Create an integer, lastAnswer, and initialize it to 0.
• The 2 types of queries that can be performed on your list of sequences (seqList) are described below:
  1. Query: 1 x y
     1. Find the sequence, seq, at index ((x ^ lastAnswer) % N) in seqList.
     2. Append integer y to sequence seq.
  2. Query: 2 x y
     1. Find the sequence, seq, at index ((x ^ lastAnswer) % N) in seqList.
     2. Find the value of element y % size in seq (where size is the size of seq) and assign it to lastAnswer.
     3. Print the new value of lastAnswer on a new line.

However, I am getting a “time out error” for very large inputs.

Below is my code in Swift so far:

func dynamicArray(n: Int, queries: [[Int]]) -> [Int] {
    var sequences: [Int: [Int]] = [:]
    var lastAnswer = 0
    var answerlist = [Int]()
    for query in queries {
        let queryType = query[0]
        let x = query[1]
        let y = query[2]
        switch queryType {
        case 1:
            let seq = (x ^ lastAnswer) % n
            if var sequence = sequences[seq] {
                sequence.append(y)
                sequences[seq] = sequence
            } else {
                sequences[seq] = [y]
            }
        case 2:
            let seq = (x ^ lastAnswer) % n
            let index = y % (sequences[seq])!.count
            lastAnswer = sequences[seq]![index]
            answerlist.append(lastAnswer)
        default: break
        }
    }
    return answerlist
}

Can it be further optimised?

Answer 1

You chose to represent the “list of sequences” as a dictionary

var sequences: [Int: [Int]] = [:]

and here you test if a sequence for the given index already exists, and then either append a new element or create the initial sequence for that index:

if var sequence = sequences[seq] {
    sequence.append(y)
    sequences[seq] = sequence
} else {
    sequences[seq] = [y]
}

That can be greatly simplified by using a subscript with a default value:

sequences[seq, default: []].append(y)

But actually, instead of representing the “list of sequences” as a dictionary I would represent it as an array (of arrays) instead:

var sequences: [[Int]] = Array(repeating: [], count: n)

The number of sequences is a-priori known, so that there is no advantage of using a dictionary. Appending a new element to a sequences (query type 1) then becomes

sequences[seq].append(y)

and for query type 2 we get

lastAnswer = sequences[seq][index]
answerlist.append(lastAnswer)

without the need of forced-unwrapping for the dictionary subscripting.

This should also be more efficient, because (array) index lookups are faster than dictionary lookups.

Some more remarks:

• The type of a variable should not be part of the variable name, e.g. answers instead of answerList.

• Multiple (related) assignments can be combined to a tuple assignment, e.g.

  let (queryType, x, y) = (query[0], query[1], query[2])
  

• We know that the query type can only be 1 or 2, but a fatalError() in the default case helps to find programming errors.

Putting it together, the function could look like this:

func dynamicArray(n: Int, queries: [[Int]]) -> [Int] {
    var sequences: [[Int]] = Array(repeating: [], count: n)
    var lastAnswer = 0
    var answers = [Int]()
    for query in queries {
        let (queryType, x, y) = (query[0], query[1], query[2])
        switch queryType {
        case 1:
            let seq = (x ^ lastAnswer) % n
            sequences[seq].append(y)
        case 2:
            let seq = (x ^ lastAnswer) % n
            let index = y % sequences[seq].count
            lastAnswer = sequences[seq][index]
            answers.append(lastAnswer)
        default:
            fatalError("Bad query type")
        }
    }
    return answers
}

---

In addition, it seems that most of the time is spent while reading the input data into the queries array, done by the (HackerRank supplied template) code

guard let queriesRowTemp = readLine()?.replacingOccurrences(of: "\\s+$", with: "", options: .regularExpression) else { fatalError("Bad input") }

let queriesRow: [Int] = queriesRowTemp.split(separator: " ").map {
    if let queriesItem = Int($0) {
        return queriesItem
    } else { fatalError("Bad input") }
}

Instead of removing trailing whitespace with a regular expression search we can split the input row and ignore empty components:

guard let queriesRowTemp = readLine() else { fatalError("Bad input") }
let queriesRow: [Int] = queriesRowTemp.split(separator: " ",
                                             omittingEmptySubsequences: true).map {
    if let queriesItem = Int($0) {
        return queriesItem
    } else { fatalError("Bad input") }
}

In my test that cut the time to read 100000 queries down from 3.7 to 1.7 seconds.