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Getting started with DSA and competitive coding
Updated on Sep, 2026
Programming, Software Engineering, Data Structures
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These notes contain basic details such as how to declare, initiate, access(add/remove), iterate arrays, set, list, map, queue, stack
stores values of the same data type. The address of elements 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 Arrays
Sorting an element — Arrays. sort(nums);
Finding the size of array — array_name.length
Filling array element with some values -
Arrays. fill(nums, -1) -> this will assign all values in the array as -1
Let us consider we loop the elements in an integer array, we can do it like 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 is written inside “ ”.
Strins s = “Welcome”; // string literal
String s=new String(“Welcome”); // string object
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 particular substring — contains()
Converting string to char array — toCharArray()
taking substrings — substring()
Replacing in string — replace()
Replacing first occurence — replaceFirst()
joining two strings — string1. concat(string2)
getting index of a character — string.indexOf(character)
removes beginnind 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 particular index
string. delete(startIndex, endIndex) — deleted string from the start to 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));
They store in the order we provide and they can have duplicates
List varName = new ArrayList<>();
List varName = new LinkedList<>();
List a1 = new ArrayList(); a1. add(“Zara”); a1. add(“Mahnaz”); a1. add(“Ayan”); System. out. println(“ ArrayList Elements”); System.out.print(“\\t” + a1);
ArrayList Elements [Zara, Mahnaz, Ayan]
List l1 = new LinkedList<>(); l1. add(“Zara”); l1. add(“Mahnaz”); l1. add(“Ayan”); System. out. println(); System. out. println(“ LinkedList Elements”); System.out.print(“\\t” + l1);
output: LinkedList Elements [Zara, Mahnaz, Ayan]
Built-in methods in List- add(value), add(index, value),
contains(value) returns true or false isEmpty() return true or false, clear(), size() returns int removeLast() removeFirst()
Difference between Array List & Linked List:
Array List stores values sequentially, so if we add or remove a value it needs more time to shift its elements and adjust
LinkedList stores randomly in memory and links by pointers. inserting and deleting takes less time here
Set can’t have duplicates
HashSet — stores values in random order and it is unsorted
LinekedHashset — stores in order we give Treeset — stores in asc order
When we want to find union/intersection/difference in two sets of data, we can use sets.
Whenever we want to find a missing or a duplicate element we can first about Sets
Declaration and initialisation:
Set set = new HashSet<>();
add(value) — returns t/f, remove(value), contains(value) — returns t/f, isEmpty() return true or false, clear(), size() returns int
Union — set1. addAll(set2);
Intersection — set1. retainAll(set2);
Difference between sets — set1. removeAll(set2);
stores unique key, value pair
Hash Map — stores in random order and the key won’t be sorted Linked Hash Map — stores in order we provide Tree Map — stores in key’s ascending order
Whenever we want to count the frequency of something, we can use Map
Whenever we want to group something and count them, we can use Map
Declaration and Initialization
Map m1 = new HashMap(); m1. put(“Zara”, “8”); m1. put(“Mahnaz”, “31”); m1. put(“Ayan”, “12”); m1. get(“Zara”);
Methods: -put(key,value), get(key) -containsKey(key) — returns true or false -containsValue(value) — returns true or false -isEmpty() — returns True or false -size() returns int -values() returns list of values -keys() returns list of keys -putIfAbsent(key,new value) -getOrDefault(existing,inititalize value) -ceilingKey(key) -> gives greater element to the key -lowerKey or floorkey(key) -> gives smaller element to the key
for(int value : map. values()){
for(int key : map. keys()){
for(Map.Entry entry : map.entrySet()){ System.out.println(“Key = “ + entry.getKey() + “, Value = “ + entry . getValue()); }
Stack stores elements of the same data type.
We can insert, and remove elements in the stack in the Last In First Out policy (LIFO)
Stack variable_name = new Stack<>();
Stack stack = new Stack<>();
Methods used in Stack:
stack_name . push() — insert element into a stack
stack_name . peek() — checks the topmost element in the stack
stack_name . pop() — removes the topmost element in the stack
stack_name . size() — returns the number of elements in the stack
stack_name . empty() — returns true / false. if the stack is empty, it returns true. if the stack is not empty it returns false
stack_name . clear() — clears the stack
stack_name . contains(value) — returns true if the value we check is there in a stack or else it returns false stack_name . remove(index) — removes the value in a given index
stack_name . search(value) — searches the value in the stack and returns us the index of the value if present.
We can go through the stack elements in two ways.
1. Popping the elements
We can pop the stack elements one by one and go through all the elements.
We can go through the stack by a for loop as below
for(String s : stack_name)
//Stack will be iterated from bottom to top direction in this way
How to identify if that problem can be solved using a stack
1. when we want to evaluate brackets, and expressions in a certain order we can use stacks.
2. When we want to use backspace characters on the keyboard or in any similar situations, we can use stacks
3. When we want to delete or remove the last elements we can use the stack.
4. When we want to backtrack to something and then again move forward direction multiple times we can use the stack.
5. We can use stack to find some peak elements (assuming the value plotted something like a graph) we can use stacks
Queue: (Interface and not class)
FIFO policy 1. Queue ll = new LinkedList(); //stores in order we insert 2. Queue pQueue = new PriorityQueue(); //it stores in Asc order. This can also be used as Min heap
2. Queue pQueue = new PriorityQueue((a,b) -> b-a); //it stores in desc order. This can also be used as max heap
methods: offer(element) used to initialize the queue add(element),
remove(element) /poll() returns top element peek()/element() returns the top element that will be polled
It is good to know Core Java
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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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