Recursion in Data Structure: How Does it Work, Types & When Used

# Recursion in Data Structure: How Does it Work, Types & When Used

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29th Sep, 2022
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Breaking big problems into smaller and more manageable pieces is what recursion in stack in data structure is all about. It is a great problem-solving method if the problem has several possible branches of solution and is too difficult to handle for an iterative approach. If reading several recursion in data structure pdf online has not been helpful enough to know what is recursion and what are the properties of recursion, then keep on reading!

Let us start with the definition of recursion in the data structure. We will later discuss different types of recursion and how recursion is used to solve different problems.

## What is recursion?

In simple words, recursion is a problem solving, and in some cases, a programming technique that has a very special and exclusive property. In recursion, a function or method has the ability to call itself to solve the problem. The process of recursion involves solving a problem by turning it into smaller varieties of itself.

Recursion in stack in data structure is when functions call themselves directly or indirectly.

The process in which a function calls itself could happen directly as well as indirectly. This difference in call gives rise to different types of recursion, which we will talk about a little later. Some of the problems that can be solved using recursion include DFS of Graph, Towers of Hanoi, Different Types of Tree Traversals, and others.

Every step repeats itself at a smaller scale in recursion. Hence, all of them are combined to solve the problem. Recursion in the data structure is one of the most compact and improved strategies for creating function calls or deploying functions in programming. It helps you to implement many functions and algorithms efficiently, at the same time extending clarity on while executing a recursive algorithm in a code which data structure is used.

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Now that you know the answer to what is recursion, it is time to learn about the properties of recursion.

## How does recursion work?

The concept of recursion is established on the idea that a problem can be solved much easily and in lesser time if it is represented in one or smaller versions. Adding base conditions to stop recursion is another important part of using this algorithm to solve a problem.

People often believe that it is not possible to define an entity in terms of itself.  Recursion proves that theory wrong. And if this technique is carried out in the right way, it could yield very powerful results. Let us see how recursion works with a few examples. What is a sentence? It can be defined as two or more sentences joined together with the help of conjunction. Similarly, a folder could be a storage device that is used to store files and folders. An ancestor could be a parent of one and an ancestor of another family member in the family tree.

Recursion helps in defining complex situations using a few very simple words. How would you usually define an ancestor? A parent, a grandparent, or a great grandparent. This could go on. Similarly, defining a folder could be a tough task. It could be anything that holds some files and folders that could be files and folders in their own right, and this could again go on. This is why recursion makes defining situations a lot easier than usual.

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Recursion is also a good enough programming technique. A recursive subroutine is defined as one that directly or indirectly calls itself. Calling a subroutine directly signifies that the definition of the subroutine already has the call statement of calling the subroutine that has been defined.

On the other hand, the indirect calling of a subroutine happens when a subroutine calls another subroutine, which then calls the original subroutine. Recursion can use a few lines of code to describe a very complex task. Let us now turn our attention to the different types of recursion that we have already touched upon.

Many programming languages offer recursive functions to directly or indirectly call themselves by calling another function that ultimately calls the initial function. The memory that is assigned whenever a function is called upon is again relocated in addition to the memory assigned to the calling function. The local variable’s duplicates are created for every single call.

The functions will return their value to the functions via which they are called. This occurs the moment the root issue is developed. Subsequently, the memories are de-allocated, and this cycle iterates. The working of recursion helps you to easily understand which data structure is used in recursion.

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The fundamental rules of Recursion:

All recursive algorithms follow these three rules:

• A recursive algorithm has to call itself recursively.
• A recursive algorithm needs a base case.
• A recursive algorithm should change its move and mention the base case.

Therefore, to sum it up, some of the basic properties of recursion algorithms include

1. A recursion algorithm will always have a base case. The base case is nothing but the condition that helps initiate the process of returning to the original call function. It is also the condition because of which the algorithm is allowed to stop the recursion process. This process is also widely known as winding the stack. It is a crucial part of performing recursion in stack data structure. More precisely, the base case, as the name suggests, is a small enough problem that can be solved directly.

On that note, in some algorithms, such as accumulate, the base case is considered an empty vector.

1. To move toward the base case, the recursion algorithm must change its state. It is important that things be modified on subsequent calls so they can be moved closer to the base. Changing the state precisely means giving the algorithm modified data to use. In most cases, the data that indicates the problem gets smaller and smaller the closer it gets to the base. In algorithms like accumulate, where the primary data structure is a vector, focusing on state-changing efforts on the vector is very important.
2. The recursive algorithm has to call itself. This is one of the properties of recursion that can be understood from the very definition of a recursive algorithm. The great thing about recursion is that it divides the big problem into smaller parts and then the programmer can solve each of them individually by writing a function for those smaller problems. However, when we have to solve a problem with functions, the function has to solve it by calling itself.

## What is a Recursive Algorithm?

Simply put, a recursive algorithm is a type of algorithm that calls itself in smaller input values. By executing fundamental functions on the returned values for the input values it returns outcomes for the present input.

If a larger problem can be solved by splitting it into several smaller tasks and accomplishing them, then the problem can be fixed by implementing a recursive algorithm.

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Recursion example in data structure can be found in solving both time and space complexities. However, there is a heavy argument regarding using recursion when there is iteration present to perform the task, one must remember that recursion is more efficient than iteration in multiple ways.

First of all, recursion makes the program more readable by breaking it into smaller chunks, and secondly, the latest enhanced CPU systems work better with recursive algorithms than iteration one.

In solving time complexities, everything is assumed to be constant, very much similar to iterations. To measure this complexity, the number of times a recursive call is being made is counted. On the other hand, in the case of calculating space complexity as a recursion example in data structure, the amount of extra space required for a module to execute is measured.

As in the methodology of excursion, the system is required to store an activation record each time a recursive call is made, which is not the case with iterations, the space complexity of the recursive function goes higher than that of iteration one. Going through a detailed recursion in data structure pdf will allow you to learn more about these recursion properties.

## Types of recursion

There are only two types of recursion as has already been mentioned. Let us see how they are different from one another. Direct recursion is the simpler way as it only involves a single step of calling the original function or method or subroutine. On the other hand, indirect recursion involves several steps.

The first call is made by the original method to a second method, which in turn calls the original method. This chain of calls can feature a number of methods or functions. In simple words, we can say that there is always a variation in the depth of indirect recursion, and this variation in depth depends on the number of methods involved in the process.

Direct recursion can be used to call just a single function by itself. On the other hand, indirect recursion can be used to call more than one method or function with the help of other functions, and that too, a number of times. Indirect recursion doesn’t make overhead while its direct counterpart does.

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Let’s understand direct and indirect recursion in detail.

1) Direct Recursion:

There may be a call to a method in the body of the same method. This method is said to be directly recursive.

Three types of Direct Recursion are:

1. Linear Recursion
2. Binary Recursion
3.  Multiple Recursion
1. Linear Recursion

Linear recursion starts its execution by testing for base cases’ sets. It then performs a single recursive call. This step involves a test that determines which recursive call to make.  It, therefore, gives you an answer on which data structure is used in recursion. This form of recursion defines every possible recursion call to ensure that it marks progress towards a base case.

2.Binary Recursion

Binary recursion takes place when there are two recursive calls for every non-base case.​​​​​​​

3. Multiple Recursion

It involves making many recursive calls

2) Indirect Recursion:

It happens when functions call some other functions to call the initial function. It includes two easy steps when creating a recursive call. It leads to making functions call functions to make a recursive call. The mutual recursion is called an indirect recursion. Understanding these types of recursion helps you to determine which data structure is used to handle recursion in C.

## When is recursion used?

There are situations in which you can use recursion or iteration. However, you should always choose a solution that appears to be the more natural fit for a problem. A recursion is always a suitable option when it comes to data abstraction. People often use recursive definitions to define data and related operations.

And it won’t be wrong to say that recursion is mostly the natural solution for problems associate with the implementation of different operations on data. However, there are certain things related to recursion that may not make it the best solution for every problem. In these situations, an alternative like the iterative method is the best fit.

The implementation of recursion uses a lot of stack space, which can often result in redundancy. Every time we use recursion, we call a method that results in the creation of a new instance of that method. This new instance carries different parameters and variables, which are stored on the stack, and are taken on the return. So while recursion is the more simple solution than others, it isn’t usually the most practical.

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Also, we don’t have a set of pre-defined rules that can help choose iteration or recursion for different problems. The biggest benefit of using recursion is that it is a concise method. This makes reading and maintaining it easier tasks than usual. But recursive methods aren’t the most efficient methods available to us as they take a lot of storage space and consume a lot of time during implementation.

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Keeping in mind a few things can help you decide whether choosing a recursion for a problem is the right way to go or not. You should choose recursion if the problem that you are going to solve is mentioned in recursive terms and the recursive solution seems less complex.

You should know that recursion, in most cases, simplifies the implementation of the algorithms that you want to use. Now if the complexities associated with using iteration and recursion are the same for a given problem, you should go with iteration as the chances of it being more efficient are higher.

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Key recursion techniques in data structure methods:

Programmers can use five recursion techniques in functional programming. Being familiar with them helps them to know which data structure is used in recursion. They are discussed below.

• Tail Recursion:

Linear recursion is till now the most extensively used recursion technique, and hence it is quite advantageous. It calls the recursive functions in the end.

• Binary Recursion:

It calls functions twice. It is widely used in operations like tree traversal and merging.

• Linear Recursion:

It is the most prevalent recursion method.  It allows functions to call themselves in a non-complex format and subsequently terminate the function through a termination condition. This strategy includes functions that make one-off calls to themselves during the implementation.

• Mutual Recursion:

It allows a function to use other functions recursively. So, this technique implies that functions call each other. It is widely used when writing programs using programming languages that frequently don’t support recursive calling functions. So, executing mutual recursion can work as a substitute for a recursion function. The starting circumstances are used for a single, several, or all of the functions. Understanding mutual recursion and other such types discussed in this section give you a clear perspective on which data structure is used to handle recursion in C

• Nested Recursion:

These types of recursions can’t be converted into an iterative format. The recursive functions pass the parameters as recursive calls which convert to recursions in recursions.

How memory allocation occurs in the Recursive Method?

Each recursive call employs a new method in the memory. When this method returns the data, the copy is extracted from the memory. This is because all the variables and other data declared in the function are saved in the stack. Consequently, a distinct stack is maintained at every recursive call. After the corresponding function returns a value, the stack will be removed.

Recursion in data structure features a high level of complexity for solving and supervising the values at every recursive call. Hence, you have to maintain the stack and supervise the variables’ values agreed in the stack. Understanding memory allocation helps you to determine which of the following data structures finds its use in recursion?

Recursion consumes more memory because the recursive function is added to the stack with every recursive call. The values are stored there until the call completes. The recursive function employs a LIFO (last in first out) structure similar to the stack data structure.

The memory is allocated to a function on the stack when it is called from the main(). When a recursive function calls that one, a called function’s memory is assigned on top of the memory assigned to the calling function. Moreover, a distinct copy of the local variables is prepared for every function call. Once the base case is achieved, the function returns its value to the function that is called upon. Finally, the memory is de-allocated and this process continues.

How to Use Recursion in Data Structure?

You may have decided on which data structure is used to handle recursion in C. But you should also know how to efficiently use recursion in data structure using various programming languages. The relevant knowledge helps you to develop a few easy programs or derive solutions to different problems to understand while executing a recursive algorithm in a code which data structure is used.

How recursion solves a particular problem?

It is vital to define a problem in the context of one or multiple smaller problems and add one or multiple base conditions that terminate the recursion. You can determine which of the following data structures finds its use in recursion?, when you understand how it solves a specific problem. For example, you can calculate factorial n if you know the factorial of (n-1). The factorial’s base case will be n = 0. It returns 1 when n = 0.

## Conclusion

However, there could be situations in which making a decision may not be that easy. You have to choose between efficiency and simplicity. If you are an experienced designer, you would know exactly when to give more importance to efficiency and when choosing simplicity or conciseness over it is the way to go.

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#### Rohit Sharma

Blog Author
Rohit Sharma is the Program Director for the UpGrad-IIIT Bangalore, PG Diploma Data Analytics Program.
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#### Data Science Skills to Master

1What is the most common real-life application of recursion?

Recursion is a process in which the function calls itself indirectly or directly in order to solve the problem. The function that performs the process of recursion is called a recursive function. There are certain problems that can be solved pretty easily with the help of a recursive algorithm.

The most common real-life application of recursion is when you are calculating how much money you have in a box filled with Rs. 100 notes. If there are too many notes, then you might just ask your friend to do the same work by dividing the entire stack into two. Once you both are done with counting, you will just add up both the results in order to get the total amount.

2What are the properties that a recursive function must have?

A recursive function has the capability to continue as an infinite loop. There are two properties that have to be defined for any recursive function to prevent it from going into an infinite loop. They are:

1. Base criteria – There has to be one predefined base condition. Whenever this base criterion is fulfilled, the function will stop calling itself.
2. Progressive approach – The recursive calls should consist of a progressive approach. Whenever a recursive call is made to the function, it should be reaching near the base condition.

Some of the advantages of recursion are that you only need to define the base condition and the recursive case in a recursive function. This makes the code pretty simple and short as compared to an iterative code, and some problems like Tree Traversal and Graph are inherently recursive.

The biggest disadvantages of recursion are that there are greater space requirements for recursive functions as compared to an iterative program because every function call has to remain in the stack until the base condition is met, and there are greater time requirements, too, because the stack grows after each function call, and the final answer can only be returned after popping all the elements from the stack.

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