Memorandum No.2 "Making a search history with ArrayList and HashSet" [Java]

Introduction

Recorded because I had a hard time in the introduction to paiza's skill check.

What you want to do

1. Enter multiple character strings of any number of characters and have them in the list.
2. At that time, if the entered character string already exists in the list, delete the previous character string and add a new one.
3. Output from the top in order from the most recently input.

Code 1

    Scanner sc = new Scanner(System.in);
    List<String> words = new ArrayList<>();
    while(sc.hasNext()) {
        String s = sc.nextLine();

if (words.contains (s)) {// Check if it's already in the list and delete it if it exists words.remove(s); } words.add (0, s); // Add the entered string to the beginning of the list }

    for(String word : words) {
        System.out.println(word);
    }

Problems with code 1

-If you increase or decrease the value of ArrayList with remove () or add (), all the stored values must be shifted, so it takes time to process. -Since contains () of ArrayList also searches all the values in the list, the more the stored values, the longer the processing time.

solution

-Use HashSet to search for character strings. → Elements can be converted to hash values and used for searching, and values can be found quickly.

Code 2

Add all the strings entered to the list and use HashSet to hide duplicates. (I don't actually delete it, I assume it wasn't there)

    Scanner sc = new Scanner(System.in);

    List<String> words = new ArrayList<>();
    while(sc.hasNext()) {
        String s = sc.nextLine();
        words.add(s);
    }

Collections.reverse (words); // Invert the list to output from the newly entered one

Set usedWords = new HashSet <> (); // HashSet used to determine duplicate strings in the list

    for(String word : words) {

if (! usedWords.contains (word)) {// Determine if it has already been added to usedWord (duplicate) and display if not System.out.println(word); } usedWords.add(word); }