The Concept

The two-pointer pattern refers to the idea of using two-pointers to efficiently traverse a linear data structure (e.g. arrays) to solve a given problem. The two-pointers typically start at different points (e.g. the beginning and end of a list) and move across the data structure in opposite directions, given that a specific condition is met.

Problems that the pattern solves

A coding problem can be solved using this pattern if it:

• Requires finding a pair of elements that satisfy a given condition e.g. finding two elements in an array whose sum equates to a given value
• Involves a reversal of elements e.g. reversing an array
• Requires you to process two elements at a time in a given data set e.g. palindrome check

Pre-requisites to using the pattern

To use this pattern, the following checks need to be met:

• The elements in the data structure need to be sorted. This helps improve time complexities, but does not apply to all problems
• Two elements should be processed at the same time in a data set e.g. comparing values in an array

The Code

We can leverage this pattern, for example, to check whether a word is a palindrome. We're techies, not literature majors, so let me educate you on what a palindrome is 😄. A palindrome is a word that is spelled the same forward and backward e.g. Abba, Eve, Anna.

The Problem

Given a string word, return true if it’s a palindrome, and false if it is not.

Explanation

• We’ll first initialize the first pointer at the beginning of the string, and the other pointer at the end of the string

• Next, we’ll check if the two values match.

  • If they match, we’ll move the start pointer to the right, and the last pointer to the left, one time each

  

  • if they do not match, we return false and terminate the flow