Pattern-first. Java. No fluff.

Crack Google, Amazon and every startup that asks DSA.

Stop grinding 500 random problems. Learn 13 patterns in order, and for every question see the brute force → better → best path, the exact time and space, and the Java you pick up along the way.

Open the sheet → Read the notes

Plain, explicit JavaEvery solution compiled and testedSpaced revision built in

TwoSum.java

// Best: remember what you've seen
static int[] twoSum(int[] nums, int target) {
    Map<Integer, Integer> seen = new HashMap<>();
    for (int i = 0; i < nums.length; i++) {
        int need = target - nums[i];
        if (seen.containsKey(need)) {
            return new int[]{seen.get(need), i};
        }
        seen.put(nums[i], i);
    }
    return new int[]{};
}

BruteO(n²)

BetterO(n log n)

BestO(n)

0curated problems

0patterns, easy to hard

0pattern blocks with full notes

0Java solutions, all tested

How every question is taught

Three solutions. One idea each.

Interviewers want to see you reach the good answer, not recite it. So each note walks the same road you would walk in the room.

Brute force

The obvious thing

Try everything. It is slow, but it is correct, and it is how you say "I understand the problem" out loud.

e.g. O(n²) nested loops

Better

Spot the waste

Find what the brute force repeats, then remove it with sorting, a hash map or a prefix sum.

e.g. O(n log n) sort + scan

Best

The interview answer

The optimal approach with code you can explain line by line, plus the time and space, plus why it works.

e.g. O(n) one pass

Java you learn

Small language facts that each question teaches: overflow, Integer ==, shallow clones.

Collection costs

The big-O of every API used, so HashMap vs TreeMap stops being a guess.

Other approaches

Kadane, sliding window, union-find, quickselect. Techniques from other topics, because those questions won't repeat.

Spaced revision

Tick a problem as done and it comes back on day 3 and day 10.

The roadmap

13 patterns, in the order that makes sense.

Work top to bottom, two problems a day. Each block starts with how to spot the pattern. Blocks marked Live have full notes. The rest are coming.

Why this works

Built for the way interviews are actually run.

🧭

Pattern first

You see a clue, you know the family of solutions. That beats memorising 500 answers.

✅

Code that ran

Every solution in the notes was compiled and cross-checked on random inputs against the brute force.

🔁

Revision built in

Progress, notes and revision dates live in your browser. Back it up with one click.

Two problems a day. Eleven weeks.

Open the sheet, pick the first unticked problem, and read its note before you look at any solution.

Open the sheet →Try a note: Two pointers