Why Learn DSA? A Tale of Two Librarians

Meet two librarians

A library has 10,000 books. Two librarians are asked: “Do we have the book Java Basics?”

• Librarian A keeps the books in one big pile. She checks the books one by one. On a bad day she checks all 10,000.
• Librarian B keeps the books on shelves sorted A to Z. She walks to the “J” shelf and finds it in seconds.

Both are correct, but B is much faster. The only difference is how the books are stored and the steps she follows. That's what DSA is about:

Let Java count the work

Here we look for one number among 1,000 numbers. We count how many numbers each method has to look at:

public class Main {
    public static void main(String[] args) {
        int[] books = new int[1000];
        for (int i = 0; i < books.length; i++) books[i] = i * 3;   // 0, 3, 6, 9 ... (sorted)
        int wanted = 2994;

        // Librarian A: check every book from the start
        int checksA = 0;
        for (int b : books) {
            checksA++;
            if (b == wanted) break;
        }

        // Librarian B: books are sorted, so look in the middle and throw away half each time
        int checksB = 0, lo = 0, hi = books.length - 1;
        while (lo <= hi) {
            checksB++;
            int mid = (lo + hi) / 2;
            if (books[mid] == wanted) break;
            if (books[mid] < wanted) lo = mid + 1;
            else hi = mid - 1;
        }

        System.out.println("Librarian A checked " + checksA + " books");
        System.out.println("Librarian B checked " + checksB + " books");
    }
}

Output

Librarian A checked 999 books
Librarian B checked 9 books

Same answer, a tiny fraction of the work. You'll learn Librarian B's trick (binary search) in Module 7.

How this course works

• Every module has short lessons and then challenges you solve in the browser.
• We often build a data structure ourselves as a small Java class first, so you see how it works inside. Then we use Java's ready-made version (ArrayList, HashMap, ArrayDeque, …).
• Every example on these pages was really run – the output you see is the real output.

Tip: You should know basic Java: variables, if, loops, methods, arrays and a simple class. If not, take the Learn Java course first – it's in the same category.