Here’s a list of all the sources which I have referenced when writing the tutorials (the same links are also provided under each tutorial):

Arrays

• Arrays (The Java Tutorials)
• Array Data Structure - GeeksforGeeks
• For-each loop in Java - GeeksforGeeks

ArrayList

• ArrayList (Java SE 11 & JDK 11)
• ArrayList in Java - GeeksforGeeks

Recursion

• Factorial - Wikipedia
• Recursion - GeeksforGeeks

OOP

• Controlling Access to Members of a Class (The Java Tutorials)
• Object-oriented programming - Wikipedia
• Object Oriented Programming (OOPs) Concept in Java - GeeksforGeeks

Searching

• Binary Search - GeeksforGeeks
• Searching Algorithms - GeeksforGeeks
• Time complexity - Wikipedia

Sorting

• Bubble Sort - GeeksforGeeks
• Bubble sort in 2 minutes - YouTube
• Insertion Sort - GeeksforGeeks