Refactored Solution In Swift

问题

I've been studying for a coding exam by doing the HackerRank test cases, for the most part I've been doing well, but I get hung up on some easy cases and you all help me when I can't see the solution. I'm working on this problem:

https://www.hackerrank.com/challenges/ctci-ransom-note

A kidnapper wrote a ransom note but is worried it will be traced back to him. He found a magazine and wants to know if he can cut out whole words from it and use them to create an untraceable replica of his ransom note. The words in his note are case-sensitive and he must use whole words available in the magazine, meaning he cannot use substrings or concatenation to create the words he needs.

Given the words in the magazine and the words in the ransom note, print Yes if he can replicate his ransom note exactly using whole words from the magazine; otherwise, print No.

Input Format

The first line contains two space-separated integers describing the respective values of (the number of words in the magazine) and (the number of words in the ransom note). The second line contains space-separated strings denoting the words present in the magazine. The third line contains space-separated strings denoting the words present in the ransom note.

Each word consists of English alphabetic letters (i.e., to and to ). The words in the note and magazine are case-sensitive. Output Format

Print Yes if he can use the magazine to create an untraceable replica of his ransom note; otherwise, print No.

Sample Input

6 4
give me one grand today night
give one grand today

Sample Output

Yes
Explanation

All four words needed to write an untraceable replica of the ransom note are present in the magazine, so we print Yes as our answer.

And here is my solution:

import Foundation

func main() -> String {
    let v = readLine()!.components(separatedBy: " ").map{Int($0)!}
    var a = [String](); var b = [String]()
    if v[0] < v[1] { return "No"}
    for i in 0 ..< 2 {
        if i == 0 {
            a = (readLine()!).components(separatedBy: " ")
        } else { b = (readLine()!).components(separatedBy: " ") }
    }

    // Get list of elements that intersect in each array
    let filtered = Set(a).intersection(Set(b))
    // Map set to set of Boolean where true means set a has enough words to satisfy set b's needs
    let checkB = filtered.map{ word in reduceSet(b, word: word) <= reduceSet(a, word: word) }

    // If mapped set does not contain false, answer is Yes, else No
    return !checkB.contains(false) ? "Yes" : "No"
}
func reduceSet(_ a: [String], word: String) -> Int {
    return (a.reduce(0){ $0 + ($1 == word ? 1 : 0)})
}

print(main())

I always time out on three of the 20 test-cases with this solution. So the solution seems to solve all the test cases, but not within their required time constraints. These are great practice, but it's so extremely frustrating when you get stuck like this.

I should note that I use Sets and the Set(a).intersection(Set(b)) because when I tried mapping an array of Strings, half the test-cases timed out.

Any cleaner, or more efficient solutions will be greatly appreciated! Thank you!

回答1:

I took the leisure of making some improvements on your code. I put comments to explain the changes:

import Foundation

func main() -> String {
    // Give more meaningful variable names
    let firstLine = readLine()!.components(separatedBy: " ").map{Int($0)!}
    let (magazineWordCount, ransomNoteWordCount) = (firstLine[0], firstLine[1])

    // a guard reads more like an assertion, stating the affirmative, as opposed to denying the negation.
    // it also 
    guard magazineWordCount > ransomNoteWordCount else { return "No" }

    // Don't use a for loop if it only does 2 iterations, which are themselves hardcoded in.
    // Just write the statements in order.
    let magazineWords = readLine()!.components(separatedBy: " ")
    let ransomNoteWords = readLine()!.components(separatedBy: " ") //You don't need ( ) around readLine()!

    let magazineWordCounts = NSCountedSet(array: magazineWords)
    let ransomNoteWordCounts = NSCountedSet(array: ransomNoteWords)

    // intersect is a verb. you're looking for the noun, "intersection"
    // let intersection = Set(a).intersection(Set(b))
    // let check = intersect.map{ countB.count(for: $0) <= countA.count(for: $0) }

    // You don't actually care for the intersection of the two sets.
    // You only need to worry about exactly the set of words that
    // exists in the ransom note. Just check them directly.
    let hasWordWithShortage = ransomNoteWordCounts.contains(where: { word in
       magazineWordCounts.count(for: word) < ransomNoteWordCounts.count(for: word)
    })

    // Don't negate the condition of a conditional expression. Just flip the order of the last 2 operands.
    return hasWordWithShortage ? "No" : "Yes"
}
print(main())

with the comments removed:

import Foundation

func main() -> String {
    let firstLine = readLine()!.components(separatedBy: " ").map{Int($0)!}
    let (magazineWordCount, ransomNoteWordCount) = (firstLine[0], firstLine[1])

    guard magazineWordCount > ransomNoteWordCount else { return "No" }

    let magazineWords = readLine()!.components(separatedBy: " ")
    let ransomNoteWords = readLine()!.components(separatedBy: " ")

    let magazineWordCounts = NSCountedSet(array: magazineWords)
    let ransomNoteWordCounts = NSCountedSet(array: ransomNoteWords)

    let hasWordWithShortage = ransomNoteWordCounts.contains{ word in
       magazineWordCounts.count(for: word) < ransomNoteWordCounts.count(for: word)
    }

    return hasWordWithShortage ? "No" : "Yes"
}
print(main())

It's simpler, and much easier to follow. :)

回答2:

Thanks to @Alexander - I was able to solve this issue using NSCountedSet instead of my custom reduce method. It's much cleaner and more efficient. Here is the solution:

import Foundation

func main() -> String {
    let v = readLine()!.components(separatedBy: " ").map{Int($0)!}
    var a = [String](); var b = [String]()
    if v[0] < v[1] { return "No"}
    for i in 0 ..< 2 {
        if i == 0 {
            a = (readLine()!).components(separatedBy: " ")
        } else { b = (readLine()!).components(separatedBy: " ") }
    }
    let countA = NSCountedSet(array: a)
    let countB = NSCountedSet(array: b)
    let intersect = Set(a).intersection(Set(b))
    let check = intersect.map{ countB.count(for: $0) <= countA.count(for: $0) }
    return !check.contains(false) ? "Yes" : "No"
}
print(main())

Many thanks!

来源：https://stackoverflow.com/questions/41729865/refactored-solution-in-swift