The Pattern That Changed Everything: Two Pointers

Day 4. Let's get technical.

When I finally understood the Two Pointers pattern - like REALLY understood it - something clicked. Suddenly, maybe 20% of interview problems felt familiar.

What is Two Pointers?

At its simplest: use two indices to traverse a data structure, instead of nested loops.

But that's like saying "addition is combining numbers." Technically true, useless in practice.

Here's what actually matters:

The Three Flavors

1. Opposite Direction (Start/End)

Both pointers start at opposite ends, move toward each other.

When to use it:

Example: Two Sum in a sorted array

2. Same Direction (Fast/Slow)

Both pointers start at the same position, move at different speeds.

When to use it:

Example: Detect cycle in linked list

3. Sliding Window (Special Case)

Technically two pointers, but with specific window semantics.

When to use it:

The Recognition Trick

When I see a problem, I ask:

1. Is the input sorted? → Consider start/end pointers

2. Does it involve a linked list? → Consider fast/slow

3. Does it mention "subarray" or "substring"? → Consider sliding window

4. Would brute force be O(n²)? → Two pointers might make it O(n)

Practice Problems (In Order)

If you want to master this pattern, do these in order:

Level 1 (Easy):

Level 2 (Medium):

Level 3 (Hard):

The Insight That Clicked

The moment it clicked for me: Two pointers is about **eliminating possibilities efficiently**.

In Two Sum II, when the sum is too big, you know the right element is too large for ANY element on the left. So you eliminate it. One comparison eliminates n possibilities.

That's why it's O(n) instead of O(n²).

Once I understood this core insight, I could apply it to new problems I'd never seen.

That's the difference between memorizing and understanding.

Tomorrow: Why I'm building in public and sharing all of this.

— Marcus