Solving Arrays & Strings Leet code questions [Java] - Online Course

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Problem solving in Array and String

Updated on Sep, 2026

Programming, Software Engineering, Data Structures

Duration - 1.5 hours

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The array is a very basic data structure representing a group of similar elements, accessed by index. The array data structure can be effectively stored inside the computer and provides fast access to all its elements

Arrays store values of the same data type. The address of elements are stored consecutively in memory length is fixed. inserting or deleting an element in the middle is not easy

int[] nums = {1,2,4} //initializing with values int[] nums = new int[10] //initializing with size. 0 will be initialized in all indexes if we declare without variables

Accessing an element:

Array index starts from 0..length-1 int[] nums = {1,2,4} nums[0] — 1 nums[1] — 2 nums[2] — 4

Built-in methods to use in Stack

Sorting an element - Arrays.sort(nums);

Finding the size of array - array_name . length

Filling arary element with some values -

Arrays.fill(nums, -1) -> this will assign all values in the array as -1

When we need to search a particular element in an array, we can do that in two ways such as

1. Linear search - We need to traverse the array completely and check if we find the element

2. Binary search — We can do binary search only when the array is sorted. Below is the command to use binary search and find if the element is present in the array or not.

Arrays.binarySearch(array,value to find) -> returns index

Let us consider looping the elements in an integer array, we can do it like the below

for (i = 0; i < nums.length; i++) {

System.out.print(nums[i]); // accessing each element of array

for (int i : nums) {

System.out.print(i);

Strings are nothing but a sequence of character values. They will be written inside “ ”.

String s=new String(“Welcome”);

The strings created like this are not mutable. If we want to add or delete anything from this string, we need to use StringBuilder or String Buffer class only

length of a string — length()

seeing character at some position — charAt()

if a string contains a particular substring — contains()

Converting string to char array — toCharArray()

taking substrings — substring()

Replacing in string — replace()

Replacing first occurrence — replaceFirst()

joining two strings — string1.concat(string2)

getting the index of a character — string.indexOf(character)

removes beginning and ending space in string — trim()

converting lower and capital letters inside string — toUpperCase() and toLowerCase()

StringBuilder is mutable.

StringBuilder string = new StringBuilder(“Welcome”);

Built-in methods used in String Builder:

All string built-in methods can be used here as well

string.append(“strings”) — to append to a string

string.insert(startIndex,” string”) — to insert a string from a particular index

string.delete(startIndex, endIndex) — deleted string from the start to the given end index

string.reverse() — to reverse the array

for (Char c: string.toCharArray()){

System.out.println(c);

for(int i = 0; i < string.length();i++)

System.out.println(string.charAt(i));

It is good to know about Java collection frameworks such as List, set, Hashmap, etc

Check out the detailed breakdown of what’s inside the course

I am a Senior Software Engineer in PayPal. I enjoy coding, especially solving problems on Leetcode. I want to share what I know by creating a course, and I'm excited to present my course on Data Structures and Algorithms in Java. I've put a lot of effort into making my courses easy to understand, explaining concepts in the simplest way I can.

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