What do you mean by recursion?
Recursion means “solving the problem via the solution of the smaller version of the same problem” or “defining a problem in terms of itself”. It is a widely used idea in programming to solve complex problems by breaking them down into simpler ones. In this blog, we’ll go over recursion basics and help you refine an essential programming skill.
Recursion comes up in mathematics frequently, where there are many examples of expressions written in terms of themselves. Calculating the value of nth factorial and nth Fibonacci numbers is the best example of this. But recursion is just as much a programming concept!
 Finding the nth Factorial: n! = n * (n1)!
 Finding the nth Fibonacci: F(n) = F(n1) + F(n2)
Recursion in Real Life
Suppose you are standing in a long queue of people. How many people are directly behind you in the line?
Rules
 One person can see only the person standing directly in front and behind. So, one can't just look back and count.
 Each person is allowed to ask questions from the person standing in front or behind. How can we solve this problem recursively?
Solution
What is recursion in programming?
In programming terms, recursion is a function calling itself until a “base condition” is true to produce the correct output.
void function(input size)
{
base case
.. .. ...
function(smaller input size)
.. .. ...
}
int main()
{
... .. ...
function(input size)
... .. ...
}
In other words: to solve a problem, we solve a problem that is a smaller instance of the same problem, and then we use the solution to that smaller instance to solve the original problem. For a recursive algorithm to work, the smaller subproblems must eventually arrive at the base case. In simple words, any recursive algorithm has two parts:
 Base Case: a terminating condition where a function can immediately return the result. This is the smallest version of the problem for which we already know the solution.
 Recursive Structure: designing a solution to a problem via the solution of its smaller subproblems i.e. same problem but for a smaller input size. We continue calling the same problem for smaller input sizes until we reach the recursion's base condition.
A Strategy for Recursive Problem Solving
Steps of problemsolving in recursion
 Define base case: think of the smallest version of the problem and write down the solution.
 Define recursive structure: now, assume we have a function to solve a problem of the given input size. Here we need to think: how could we use the problem's solution for a smaller input size to solve the problem for the given input size?
 Combine the base case and the recursive structure to generate a complete solution to the problem.
Things to keep in mind while working with recursion
 Our code must cover all valid instances of smaller input sizes.
 We must have a correct base case that makes no recursive calls.
 When we make a recursive call, it should call a smaller instance and progress towards the base case.
 When we have a correct recursive structure and base case, then recursion would solve the problem for us. This is a "recursive leap of faith" where we should not worry about intermediate steps of the recursive calls.
Basic Examples of Recursive Functions

Calculate the sum of two numbers using recursion
sum(x, y)
= x, if(y == 0)
= 1 + sum (x, y  1), if(y > 0)

Calculate the product of two numbers using recursion
product(x, y)
= 0, if(y == 0)
= sum (x, product(x, y  1), if(y > 0)

Calculate the power of two numbers using recursion
power(x, y)
= 1, if(y == 0)
= product (x, power(x, y  1), if(y > 0)
Understanding Recursion via finding nth Factorial
The factorial of a nonnegative integer is a multiplication of all integers smaller than or equal to n. For example. the factorial of 5 is 1 * 2 * 3 * 4 * 5 = 120
Recursive structure
According to the mathematical definition of the factorial of n, we can write:
n!
= n * (n  1) * (n  2) *….* 2 * 1
= n * (n  1)!
=> nth factorial = n * (n  1)th factorial
If we calculate the value of the (n  1)th factorial, we can easily calculate the value of the nth factorial. It means we can solve the problem of input size n with its smaller problem of the input size (n  1). In other words, we can solve this problem by using the idea of recursion!
Suppose the function fact(n) and fact(n  1) return the value of the nth and (n  1)th factorial, respectively, then we can write the following recursive structure:
fact(n) = n * fact(n  1)
Base case
In every recursive solution, there must be a terminating condition or base case where our recursion will directly give us results without breaking it again into the subproblem. If we observe the above recursive structure, then we find the following chain of recursive calls behind the scene:
fact(n)
= n * fact(n  1)
= n * (n  1) * fact(n  2)
... and so on
= n * (n  1) * (n  2) * ... * 4 * 3 * 2 * fact(1)
= n * (n  1) * (n  2) * ... * 4 * 3 * 2 * 1 * fact(0)
The factorial of a negative number is not defined, so fact(0) is the smallest version of the factorial problem where our recursion will terminate and return the value directly. So n = 0 is the base case that will return the value 1.
Recursive pseudocode of nth Factorial
int fact(int n)
{
if(n == 0)
return 1
return n * fact(n  1)
}
How Does Recursion Works in the Background?
If we draw the flow of recursion for the above factorial program, one can find this pattern: we are calling fact(0) last, but it is returning the value first. Similarly, we are calling fact(n) first, but it is returning the value last. Did you find some Last In First Out (LIFO) orders for recursive calls and return values? Yes, you got it right! Behind the scene, the compiler uses a stack data structure to simulate recursion and deliver the correct output. We call this stack: Call Stack!

Order of recursive calls: larger problem to smaller problem
fact(n) > fact(n  1) > ... > fact(i) > ... > fact(1) > fact(0)

Order of return values: smaller problem to larger problem
fact(0) > fact(1) > ...> fact(i) > ... > fact(n  1) > fact(n)
How the idea of call stack work in recursion?
 The information about the execution of a recursive function is stored in the call stack. It contains details about the execution: the current state of the function control flow, local variables, and other internal information. When any function is called from main(), the memory is allocated to it on the stack.
 During the recursion, when the function calls the same function for a smaller input size, memory is allocated to it, and it goes on the top of the call stack.
 The memory for a called function is allocated on top of memory allocated to the calling function, and a different copy of local variables is created for each function call.
 When the base case is reached, the function returns its value to the function by whom it is called, memory is deallocated, and the process continues.
Recursion visualization for calculating fact(5)
Recursion visualization for calculating fact(3)
Image Source: mit.edu
Examples of Some Famous Recursive Algorithms
Reverse an array
 Recursive structure: reverse (A[], l, r) = swap(A[l], A[r]) + reverse(A, l+1, r1)
 Base case: if (l >= r) then return
 Recurrence relation: T(n) = T(n  2) + c, Time complexity = O(n)
Finding the GCD of two numbers
 Recursive structure: GCD(a, b) = GCD(b, a mod b), here a > b
 Base case: GCD(a, 0) = a
 Recurrence relation: T(n) = T(n/d) + c, where d is a decreasing factor, Time complexity = O(log b)
Finding the nth Fibonacci
 Recursive structure: fib(n) = fib(n1) + fib(n2)
 Base case: We have 2 base cases: fib(0) = 0 and fib(1) = 1
 Recurrence relation: T(n) = T(n1) + T(n2) + C, Time complexity = O(2^n)
Tower of Hanoi problem
Binary Search Algorithm
Merge Sort Algorithm
Quick Sort Algorithm
Reverse a Linked List
Postorder Traversal of a Binary Tree
Print all permutation of a given string
Stack overflow error in recursion
When we call a recursive function, the return address and arguments are pushed onto the call stack. The stack is finite, so if the recursion is too deep, you'll eventually run out of stack space. This is also called the stack overflow in recursion. In some situations, if the recursive function is tailrecursive, some compilers might optimize the recursive call away by turning it into a jump. Popular reasons for stack overflow error:
 The recursive function is written with a missing base case.
 A recursive function call with an incorrect base case.
Common mistakes in recursive implementations
Here are two common ways that a recursive implementation can go wrong:
 The base case is missing entirely, or the problem needs more than one base case, but not all the base cases are covered.
 The recursive step doesn’t reduce to a smaller subproblem, so the recursion doesn’t converge.
Look for these when you’re debugging. On the bright side, an infinite loop in an iterative implementation usually becomes a Stack Overflow Error in a recursive implementation. A buggy recursive program fails faster.
Application of recursion in problemsolving
 Divide and Conquer Approach, Decrease and Conquer
 Searching and sorting algorithms: Binary search, Merge sort, Quicksort, etc.
 Problemsolving using recursive tree traversals
 Problemsolving using DFS in a graph
 Solving Dynamic Programming problems
 Problemsolving using Backtracking
 Solving linked list problems
 Designing approximation algorithms
Concepts to explore further in recursion
 Analysis of recursion
 Types of recursion
 Tail recursion and recursive optimizations
 The idea of functional programming
Coding problems to practice in Recursion
Content references
 Algorithms by CLRS
 The Algorithm Design Manual by Steven Skiena
Enjoy learning, Enjoy coding, Enjoy algorithms!