palindrome.py

def is_palindrome_v1(s):
    """ (str) -> bool

    Return True if and only if s is a palindrome.

    >>> is_palindrome_v1('noon')
    True
    >>> is_palindrome_v1('racecar')
    True
    >>> is_palindrome_v1('dented')
    False
    """
    return reverse(s) == s


def reverse(s):
    """ (str) -> str

    Return a reversed version of s.
    
    >>> reverse('hello')
    "olleh'
    >>> reverse('a')
    'a'
    """
    rev = ''

    # For each character in s, add that char to the beginning of rev
    for ch in s:
        rev = ch + rev

    return rev 


def is_palindrome_v2(s):
    """ (str) -> bool

    Return True if and only if s is a palindrome.

    >>> is_palindrome_v2('noon')
    True
    >>> is_palindrome_v2('racecar')
    True
    >>> is_palindrome_v2('dented')
    False
    """
    #The number of chars in s.
    n = len(s)

    # Compare the first half of s to the reverse of the second half.
    # Omit the middle character of an odd length string.
    return s[:n // 2] == reverse(s[n - n // 2:])


def is_palindrome_v3(s):
    """ (str) -> bool

    Return True if and only if s is a palindrome.

    >>> is_palindrome_v3('noon')
    True
    >>> is_palindrome_v3('racecar')
    True
    >>> is_palindrome_v3('dented')
    False
    """

    i = 0
    j = len(s) - 1

    while i < j and s[i] == s[j]:
        i = i + 1
        j = j - 1
        
    return j <= i