TheAlgorithms-Ruby/data_structures/arrays/strings/palindrome.rb

# Challenge name: Valid Palindrome
#
# Given a string s, determine if it is a palindrome,
# considering only alphanumeric characters and ignoring cases.
#
# Example 1:
# Input: s = "A man, a plan, a canal: Panama"
# Output: true
# Explanation: "amanaplanacanalpanama" is a palindrome.
#
# Example 2:
# Input: s = "race a car"
# Output: false
# Explanation: "raceacar" is not a palindrome.
#
# Constraints:
# 1 <= s.length <= 2 * 105
# s consists only of printable ASCII characters.
# @param {String} s
# @return {Boolean}

#
# Approach 1: Using Ruby method .reverse
#
# Time Complexity: O(n)
#
def is_palindrome(s)
  letters_only = s.downcase.gsub(/[^0-9a-z]/i, '')
  letters_only.reverse == letters_only
end

s = 'A man, a plan, a canal: Panama'
puts is_palindrome(s)
# Output: true
# Explanation: "amanaplanacanalpanama" is a palindrome.

s = 'race a car'
puts is_palindrome(s)
# Output: false
# Explanation: "raceacar" is not a palindrome.

s = 'ab_a'
puts is_palindrome(s)
# Output: true
# Explanation: "aba" is a palindrome.

#
# Approach 2: Reversed array
#
# Time Complexity: O(n)
#
def is_palindrome(s)
  letters_only_array = s.downcase.gsub(/[^0-9a-z]/i, '').split('')
  reversed_array = []
  letters_only_array.each do |letter|
    reversed_array.unshift(letter)
  end
  letters_only_array == reversed_array
end

s = 'A man, a plan, a canal: Panama'
puts is_palindrome(s)
# Output: true
# Explanation: "amanaplanacanalpanama" is a palindrome.

s = 'race a car'
puts is_palindrome(s)
# Output: false
# Explanation: "raceacar" is not a palindrome.

s = 'ab_a'
puts is_palindrome(s)
# Output: true
# Explanation: "aba" is a palindrome.