Merge branch 'jk-palindrome' of https://github.com/jsca-kwok/Ruby into jk-palindrome

Author: jsca-kwok

DIRECTORY.md

@@ -24,6 +24,7 @@
       * [Almost Palindrome Checker](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/strings/almost_palindrome_checker.rb)
       * [Anagram Checker](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/strings/anagram_checker.rb)
       * [Jewels And Stones](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/strings/jewels_and_stones.rb)
+      * [Palindrome](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/strings/palindrome.rb)
       * [Remove Vowels](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/strings/remove_vowels.rb)
     * [Two Sum](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/two_sum.rb)
     * [Two Sum Ii](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/two_sum_ii.rb)

data_structures/arrays/strings/palindrome.rb

@@ -0,0 +1,127 @@
+# 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
+#
+# Complexity Analysis:
+#
+# Time Complexity: O(n), in length n of the string.
+#
+# We need to iterate through the string: When we filter out non-alphanumeric characters and convert the remaining
+# characters to lower-case. When we reverse the string. When we compare the original and the reversed strings. 
+# Each iteration runs linearly in time (since each character operation completes in constant time). 
+# Thus, the effective run-time complexity is linear.
+#
+# Space Complexity: O(n), in length n of the string. We need O(n) additional
+# space to store the filtered string and the reversed string.
+#
+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.
+
+#
+# Approach 2: Two Pointers
+#
+
+#
+# Complexity Analysis:
+#
+# Time Complexity: O(n), in length n of the string. We traverse over each
+# character at most once until the two pointers meet in the middle, or when
+# we break and return early.
+# Space Complexity: O(1). No extra space required, at all.
+#
+def is_palindrome(s)
+  letters_only = s.downcase.gsub(/[^0-9a-z]/i, '')
+  p1 = 0
+  p2 = letters_only.length - 1
+
+  while p1 < p2
+    if letters_only[p1] == letters_only[p2]
+      p1 += 1
+      p2 -= 1
+    else
+      return false
+    end
+  end
+  true
+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.