DSA · Lesson 2 of 9

Arrays & two pointers

Solving array problems in one pass with two moving indexes.

The two-pointer technique keeps two indexes that move towards each other or in the same direction, avoiding a nested loop.

Opposite ends — sorted pair sum, palindrome check, reversing in place.
Slow/fast — removing duplicates from a sorted array, moving zeros to the end.

A close cousin is the sliding window: keep a window [left, right] and update a running sum or count as it slides, e.g. “maximum sum of k consecutive days”.

Try it yourself

Code editor

Press Run to see the output.

Edit and press Run (or Ctrl/⌘ + Enter). Python runs in your browser — the first run downloads the Python engine and takes a few seconds.

Key points

Two pointers turn many O(n²) pair searches into O(n).
Opposite-end pointers need sorted data.
Sliding window = add the new item, remove the old one.

Quick check

Why must the array be sorted for the opposite-ends pair-sum method?

To save memory So moving a pointer predictably increases or decreases the sum Python requires it It doesn't need to be

Practice task

Write move_zeros(nums) that moves all 0s to the end in place while keeping the order of other numbers.

Interview corner

How would you find two numbers summing to a target in an unsorted array?

Use a dict of value → index: for each x, check if target − x is already in it. O(n) time, O(n) space.

What is the sliding window pattern?

Maintaining a contiguous range and updating its state incrementally as it expands/shrinks, instead of recomputing from scratch.