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Recursion for Coding Interviews in Java

Direct vs. Indirect Recursion

Explore the concepts of direct and indirect recursion in Java. Understand how recursive methods call themselves or each other, including base and recursive cases, to solve problems effectively. This lesson prepares you to apply these techniques in coding interviews and practical scenarios.

We'll cover the following...

Direct Recursion

Direct recursion occurs when a method calls itself.

This results in a one-step recursive call: the method makes a recursive call inside its own body.

Java
class ExampleClass {
  private static void f() {
    
    // some code...
    f();
    //some code...
  }
  public static void main(String args[] ) {
    // Method called here
  }
}

The code snippet below gives an example of a direct recursive method that computes the square of a number.

Java
class Square {
    // Recursive method to calculate square of a number
    private static int square(int n) {
        // Base case
        if (n == 0) {
            return 0;
        }
        // Recursive case
        else {
            return square(n-1) + (2 * n) - 1;
        }
    }
    public static void main( String args[] ) {
        int input = 6;
        int output = square(input);
        System.out.println("The square of the number " + input + " is: " + output);
    }
}

We will now briefly discuss the two main parts of a recursive method, the base case and the recursive case, implemented in the code above.

The Base Case

We have defined the base case on line 5 where it states that when the variable n equals to 00, the method should terminate and start popping frames from the stack.

The Recursive Case

Let’s take a look at the mathematical operation required to perform n2n^2. We need to decrement the value of n in such a way that we can use it to call the same method but not change the mathematical formula. We get this:

(n−1)2 ...