SDE - 1

A simple approach would be to create a new arrays with size as sum of the sizes of both the arrays. Copy the elements of both the arrays in the new array and sort the array. 
A space optimised approach also exists. While traversing the two sorted arrays parallelly, if we encounter the jth second array element is smaller than ith first array element, then jth element is to be included and replace some kth element in the first array. 

Algorithm : 

1) Initialize i,j,k as 0,0,n-1 where n is size of arr1 

2) Iterate through every element of arr1 and arr2 using two pointers i and j respectively
if arr1[i] is less than arr2[j]
increment i
else
swap the arr2[j] and arr1[k]
increment j and decrement k

3) Sort both arr1 and arr2

Recursion can be used to reverse a stack. In this approach, we pop the top element from the given stack and recursively call another instance of the same function. When this child function returns to the parent function, append the popped element to the bottom of the stack. For this, two recursive functions can be used: reverseStack() and insertAtBottom(). 

reverseStack() :
It checks if the stack is empty or not. The top element is popped out and stored in the element variable if the stack is not empty. We then call reverseStack() function again on the updated stack. When this child function returns, we call the helper function insertAtBottom().

insertAtBottom() : 
It recursively inserts an element at the bottom of a stack. If the stack is empty, it simply pushes the element else; the top element is popped out and stored in the topElement variable. Then insertAtBottom() is called again on the updated stack. When this child function returns, it pushes the topElement back in the stack.

Time Complexity : O(N²) 
Two recursive functions are there in which the first function is calling itself recursively and the second function in each call. Also, the second function is recursively called.

Space Complexity : O(N)
The recursive program uses a memory of O(N) to store the function call stack.

It is:
A specification where working of Java Virtual Machine is specified. But implementation provider is independent to choose the algorithm. Its implementation has been provided by Oracle and other companies.
An implementation Its implementation is known as JRE (Java Runtime Environment).
Runtime Instance Whenever you write java command on the command prompt to run the java class, an instance of JVM is created.

When you use a new keyword, the JVM creates an instance for the object in a heap. While the reference of that object stores in the stack. There exists only one heap for each running JVM process. When heap becomes full, the garbage is collected.

Polymorphism in Java is a concept by which we can perform a single action in different ways. Polymorphism is derived from 2 Greek words: poly and morphs. The word "poly" means many and "morphs" means forms. So polymorphism means many forms.

There are two types of polymorphism in Java: compile-time polymorphism and runtime polymorphism. We can perform polymorphism in java by method overloading and method overriding.
If you overload a static method in Java, it is the example of compile time polymorphism. Here, we will focus on runtime polymorphism in java.

If subclass (child class) has the same method as declared in the parent class, it is known as method overriding in Java. In other words, If a subclass provides the specific implementation of the method that has been declared by one of its parent class, it is known as method overriding.

Rules for Java Method Overriding
The method must have the same name as in the parent class
The method must have the same parameter as in the parent class.
There must be an IS-A relationship (inheritance).