How to check if string1 is the reverse of string2 using recursion (python)

Question

I know how to reverse a string using recursion

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

def main() :
    r = reverse("Hello!")
    print(r)
    print("Expected: !olleH")
main()

However I'm having trouble figuring out how to check if one string is the reverse of a second given two strings. I want the output to be true if it is the reverse, false if it isn't the reverse. Capitals and lowercases ignored. This is what I have so far:

def isReversed(str1, str2):
    if len(str1) != len(str2):
        return False

I'm not sure how to call the function within itself using two arguments.

Answer 1

Since your reverse() function is good, you could compare string str1 to see if it's the reverse of str2 by:

def isReversed(str1, str2):
    return str1 == reverse(str2)

Because, if str1 and str2 are really the reverse of one another, reversing one of them should make them match.

If you really want to ignore case, then:

def isReversed(str1, str2):
    return str1.lower() == reverse(str2).lower()

You can also do an easy string reversal with str1[::-1] so you could check if str1 == str2[::-1]

Answer 2

If both strings are empty, they are clearly mirror images. Otherwise, compare first and last letters and recurse on the rest of the characters.

def helper(s1, s2):
    if s1 == "" == s2:  # short for s1 == "" and s2 == ""
        return True
    else:
        return s1[0] == s2[-1] and mirror_images(s1[1:], s2[:-1])

The above assumes the strings have the same length; it's best to check that once before you start the recursion:

def mirror_image(s1, s2):
    return len(s1) == len(s2) and helper(s1, s2)

Answer 3

>>> qq = 'xyz'
>>> ww = 'zyx'
>>> qq == ww
 False
>>> ww[::-1]
 'xyz'
>>> qq == ww[::-1]
 True

use can use lower() to take care of the case here is a fuller solution:

In [130]: def rev(x, y):
     ...:     if x == '' and y == '':
     ...:         return True
     ...:     elif x == '' or y == '':
     ...:         return False
     ...:     elif x[0].lower() != y[-1].lower():
     ...:         return False
     ...:     else:
     ...:         return rev(x[1:], y[:-1])
     ...:     
     ...:         
In [131]: rev('abc', 'cba')
Out[131]: True
In [132]: rev('abc', 'cb')
Out[132]: False
In [133]: rev('abc', '')
Out[133]: False
In [134]: rev('', '')
Out[134]: True