What is Recursion

• Recursion is a process for solving problems by subdividing a larger problem into smaller cases of the problem itself and then solving the smaller, more trivial parts.
• An algorithm can be implemented recursively if it can be expressed in smaller version of itself.

Factorial

• The factorial of a positive integer n can be used to calculate the number of permutations of n elements. The function is defined as
  • n! = n * (n -1) * (n2) *…*1
  • 6! = 65432*1

def factorial(n):
	if n==1:
		return 1
	else:
		return n * factorial(n-1)
print(factorial(6))

Fibonacci

• The Fibonacci sequence is a sequence of integer values in which the first two values are both 1 and each subsequent value is the sum of the two previous values. The first 11 terms of the sequence are..
  • 1; 1; 2; 3; 5; 8; 13; 21; 34; 55; 89; : : :
  • The nth Fibonacci number can be computed as follow

def fibo(n):
	if n <= 2:
		return 1
	else:
		return fibo(n-1) + fibo(n-2)
print(fibo(5))

Types of Recursion

• Linear
  • One function call
  • Have a linear recursion trace.
• Binary
  • Two function call
  • Have binary tree recursion trace
• Multiple
  • Multiple function call

Recursive Runtime

• When you have a recursive function that makes multiple calls, the runtime will often (but not always) look like O( branches ** depth ), where branches is the number of times each recursive call branches.
  • For example Fibonacci ( return fib(n-1) + fib(n-2))
  • Has a runtime of 2**N (since two branch)