Check If an Anagram of a String Is a Palindrome in Python

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πŸ’‘ Problem Formulation: The challenge is to determine whether there exists any permutation of a given string that forms a palindrome. A palindrome is a word that reads the same backward as forward, such as “radar” or “level”. In this context, an anagram is a rearrangement of the letters in the string. For example, given the input ‘aabbcc’, the desired output would be ‘True’ because ‘acbca’ is an anagram of a string that is also a palindrome.

Method 1: Using a Counter

This approach involves using a Counter from the collections module to count the frequency of each character. The condition for the existence of a palindromic anagram is that at most one character can have an odd frequency. We use a Counter to tally the characters and then check the frequencies to satisfy the palindromic condition.

Here’s an example:

from collections import Counter

def can_form_palindrome(s):
    return sum(v % 2 for v in Counter(s).values()) <= 1

print(can_form_palindrome("aabbcc"))

Output: True

The function can_form_palindrome() checks if a string can be rearranged to form a palindrome. It uses Python’s Counter to count the occurrences of each character in the string. It then uses a generator expression to count how many characters appear an odd number of times, which should be at most 1 for the string to be able to form a palindrome. In this example, “aabbcc” can form the palindrome “abcba”, so the function returns True.

Method 2: Using a Set for Odd Count Characters

Another way to solve the problem is by using a set to keep track of characters that have an odd count. We iterate through each character in the string and toggle the presence of the character in the set. In the end, if the set has more than one character, it means that more than one character has an odd count, and thus an anagram cannot be a palindrome.

Here’s an example:

def can_form_palindrome(s):
    odd_chars = set()
    for c in s:
        odd_chars ^= {c}
    return len(odd_chars) <= 1

print(can_form_palindrome("aabbcc"))

Output: True

The function can_form_palindrome() iterates over each character, using symmetric difference (^=) with a set containing the current character. If the character count is even, it will be removed from the set; otherwise, it will be added. In the end, if the length of the set is less than or equal to 1, we can form a palindrome, since we can have at most one character with an odd count in a palindrome.

Method 3: Direct Frequency Count

This method directly counts the occurrence of each character using a dictionary and checks the odd occurrence conditions. The objective is to ensure there is zero or one character that has an odd number of occurrences, as this is the requirement to form a palindrome through any permutation of the characters.

Here’s an example:

def can_form_palindrome(s):
    char_freq = {}
    for char in s:
        char_freq[char] = char_freq.get(char, 0) + 1
    odd_count = sum(1 for char in char_freq if char_freq[char] % 2 == 1)
    return odd_count <= 1

print(can_form_palindrome("aabbcc"))

Output: True

In this code snippet, we’re manually counting the frequency of each character in the string using a dictionary. We then calculate the odd count, which is the number of characters that appear an odd number of times. If this count is less than or equal to one, the string passes the palindrome test.

Method 4: Use of Bit Vector

This technique uses a bit vector to keep track of the parity of the character counts. It’s a more memory-efficient way when compared to a set or dictionary. We flip the corresponding bit for each character and ultimately check if there is at most one non-zero bit in the bit vector.

Here’s an example:

def can_form_palindrome(s):
    bit_vector = 0
    for char in s:
        bit_vector ^= 1 << ord(char)
    return bit_vector == 0 or bit_vector & (bit_vector - 1) == 0

print(can_form_palindrome("aabbcc"))

Output: True

Here, can_form_palindrome() uses bit manipulation to determine palindrome potential. The bit_vector represents the parity of character counts, flipping each bit for corresponding character ASCII values. The final condition checks whether there is at most one set bit, using a clever bit trick to test if bit_vector is a power of two (which includes zero).

Bonus One-Liner Method 5: Functional Approach

A one-liner solution employing a functional approach can be written using Python’s built-in functions and a lambda. This method combines filters, maps, and sum with a generator expression for an elegant solution.

Here’s an example:

print((lambda s: sum(map(lambda x: x % 2, map(s.count, set(s)))) <= 1)("aabbcc"))

Output: True

This one-liner defines a lambda function that first creates a set of unique characters, then maps the count of each unique character back to itself, applies a modulo operation to check for odd occurrences, and sums them up to ensure there is at most one odd occurrence.

Summary/Discussion

  • Method 1: Using a Counter. Strengths: Readable and concise with built-in functionality. Weaknesses: Slightly less efficient due to the overhead of the Counter object.
  • Method 2: Using a Set. Strengths: Direct and understandable logic for odd counts. Weaknesses: May be less intuitive for beginners due to bit operations.
  • Method 3: Direct Frequency Count. Strengths: Easy to understand manual counting. Weaknesses: Potentially less efficient than using built-in data structures.
  • Method 4: Use of Bit Vector. Strengths: Memory-efficient and fast for large strings. Weaknesses: Requires understanding of bit manipulation and character ASCII values.
  • Method 5: Functional Approach. Strengths: Elegant and concise one-liner. Weaknesses: Can be hard to read and understand for those not familiar with functional programming concepts.