Two Pointers: Opposite Ends Resource

Two Pointers: Opposite Ends Resource - aloalgo

Start with pointers at both ends of a sorted array. Move them toward each other based on some condition. This pattern is perfect for problems where you need to find pairs that satisfy a condition.

Find two numbers that add up to target in a sorted array.

Why it works: If the sum is too small, moving the left pointer right increases the sum. If too large, moving the right pointer left decreases it. Since the array is sorted, we're guaranteed to find the pair if it exists.

Find two lines that form a container holding the most water.

Why move the shorter line? The area is limited by the shorter line. Moving the taller line inward can only decrease the area (smaller width, same or smaller height). Moving the shorter line gives us a chance to find a taller line.

Check if a string is a palindrome, considering only alphanumeric characters.

Key insight: Compare characters from both ends moving inward. If they ever don't match, it's not a palindrome.

Find all triplets that sum to zero. Fix one element, then use two pointers on the rest.

Pattern: Reduce 3Sum to 2Sum by fixing one element and using two pointers for the remaining two. Careful duplicate handling avoids repeated triplets.

Calculate how much water can be trapped between bars.

Key insight: Water at any position is determined by the minimum of the max heights on either side. By processing from the shorter side, we always know the limiting factor.

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Two Pointers: Opposite Ends

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Two Sum (Sorted Array)

Container With Most Water

Valid Palindrome

3Sum

Trapping Rain Water

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