#### Introduction

Two pointer problems require you to use two (or more) pointers whose values shift one at a time. These problems are similar to sliding window problems but unlike sliding window problems which usually have a fixed window size that moves from right to left or left to right, two pointer problems don't usually keep a fixed window. Two pointers problems can be some of the trickest problems to intuit so it's important to put in the reps and do as many problems as possible.

#### A Couple Examples

*Problem*

Given two sorted integer arrays *nums1* and *nums2*, merge *nums2* into *nums1* as one sorted array.

The number of elements initialized in *nums1* and *nums2* are m and n respectively. You may assume that *nums1* has a size equal to m + n such that it has enough space to hold additional elements from *nums2*.

*Algorithm*

The main trick required to solve this problem is to recognize that we need to solve it from right to left so that we don't overwrite the values in *nums1. *If we have one pointer at m-1 and another at n-1, and a final pointer at m+n-1 and work from right to left comparing values one at a time and placing the larger value at the right, then once both the right and left pointer reach 0 we know we will have processed all elements and the array will be in sorted order.

*Problem*

Given an integer array nums sorted in **non-decreasing** order, return *an array of **the squares of each number** sorted in non-decreasing order*.

*Algorithm *

This problem would require an *unchanged *array if the numbers are all positive. The only hiccup comes from the fact that the smallest value in the original array could have the largest square in the new array. For example if we had [-4,2,1] the square of these values would be [1,4,16]. So we use two pointers one on the far left and one on the right hand and compare the squares of each one by one and place the largest value on the far right in a new array.

#### Practice Problems

**EASY**

**MEDIUM**

○ Longest Substring Without Repeating Characters

○ Longest Repeating Character Replacement

**HARD**