1. A recursive algorithm must have a base case.
2. A recursive algorithm must change its state and move toward the base case.
3. A recursive algorithm must call itself, recursively.

4.5. Converting an Integer to a String in Any Base

def toString(number,base):
    convertstring = '0123456789ABCDEF'
    if number < base:
        return convertstring[number]
    else :
        return toString(number//base,base) + convertstring[number%base)

reverse a string

def reverse(s):
    if len(s) == 1 or 0 :
        return s
    else:
        return reverse(s[1:])+ s[0]

check ispalindrome

def ispalindrome(s):
    if len(s) == 0 or 1 :
        return True
    else:
        s = s.strip()
        if s[0] == s[-1]:
            return ispalindrome(s[1:-1])
    else:
          return False