# What Is Recursion in Data Structure and Algorithm?

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Angela Bailey

Recursion is a fundamental concept in data structure and algorithm. It refers to the process of solving a problem by breaking it down into smaller, simpler instances of the same problem. In simple words, recursion involves a function calling itself to solve a smaller version of the problem until a base case is reached.

Why Use Recursion?
Recursion offers an elegant way to solve complex problems that can be broken down into simpler subproblems. It allows us to write concise and readable code by abstracting away the repetitive steps involved in solving the problem.

How Does Recursion Work?
To understand recursion, let’s consider an example – calculating the factorial of a number. The factorial of a non-negative integer n, denoted as n!, is the product of all positive integers less than or equal to n.

Let’s define a recursive function to calculate the factorial:

“`python
function factorial(n):
if n equals 0:
return 1
else:
return n multiplied by factorial(n minus 1)
“`

In this function, we have defined two cases – the base case and the recursive case. The base case occurs when n equals 0, where we simply return 1. The recursive case occurs when n is greater than 0, where we call the `factorial` function with `n – 1` as an argument and multiply it with n.

The recursion continues until the base case is reached (n equals 0), at which point all function calls are resolved in reverse order, leading to the final result.

Visualizing Recursion
Recursion can sometimes be difficult to understand conceptually. Let’s visualize how recursion works using an example.

Suppose we want to calculate `factorial(4)`. The function calls would look like this:

“`
factorial(4)
|
└── factorial(3)
|
└── factorial(2)
|
└── factorial(1)
|
└── factorial(0)
“`

At this point, the base case is reached, and the function calls start resolving:

“`
factorial(4) = 4 * factorial(3)
= 4 * (3 * factorial(2))
= 4 * (3 * (2 * factorial(1)))
= 4 * (3 * (2 * (1 * factorial(0))))
= 4 * (3 * (2 * 1))
= 24
“`

Recursion in Data Structures and Algorithms
Recursion is a powerful technique used in various data structures and algorithms. Some common examples include tree traversal, graph traversal, and sorting algorithms like merge sort and quicksort.

When dealing with recursive algorithms, it is essential to define the base case correctly to avoid infinite recursion. Additionally, recursive algorithms may have higher space complexity due to the function call stack.

– Simplifies complex problems by breaking them down into smaller subproblems.
– Allows for concise and readable code.
– Can handle problems with inherent recursive nature more intuitively.