Merge pull request #155 from jsca-kwok/jk-common-characters

Add common characters algorithm

DIRECTORY.md

@@ -51,6 +51,7 @@
   * Hash Table
     * [Anagram Checker](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/anagram_checker.rb)
     * [Arrays Intersection](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/arrays_intersection.rb)
+    * [Common Characters](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/strings/common_characters.rb)
     * [Find All Duplicates In An Array](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/find_all_duplicates_in_an_array.rb)
     * [Good Pairs](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/good_pairs.rb)
     * [Richest Customer Wealth](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/richest_customer_wealth.rb)

data_structures/hash_table/common_characters.rb

@@ -0,0 +1,51 @@
+# Challenge name: Find Common Characters
+#
+# Given an array A of strings made only from lowercase letters, return a list
+# of all characters that show up in all strings within the list
+# (including duplicates).  For example, if a character occurs 3 times in all
+# strings but not 4 times, you need to include that character three times in
+# the final answer.
+#
+# You may return the answer in any order.
+#
+# Example 1:
+# Input: ["bella","label","roller"]
+# Output: ["e","l","l"]
+#
+# Example 2:
+# Input: ["cool","lock","cook"]
+# Output: ["c","o"]
+
+#
+# Approach 1: Hash
+#
+# Time Complexity: O(n)
+#
+def common_characters(arr)
+  target_count = arr.count
+
+  hash = Hash.new(0)
+  (0...target_count).each do |i|
+    arr[i].split('').each do |letter|
+      hash[letter] += 1
+    end
+  end
+
+  result = []
+  hash.each do |k, v|
+    while v >= target_count
+      if v >= target_count
+        result << k
+        v -= target_count
+      end
+    end
+  end
+
+  result
+end
+
+puts common_characters(["bella","label","roller"])
+# => ["e","l","l"]
+
+puts common_characters(["cool","lock","cook"])
+# => ["c","o"]