Merge pull request #125 from TheAlgorithms/array-intersection

Arrays intersection

DIRECTORY.md

@@ -38,6 +38,7 @@
     * [Preorder Traversal](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/binary_trees/preorder_traversal.rb)
   * 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)
     * [Find All Duplicates In An Array](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/find_all_duplicates_in_an_array.rb)
     * [Richest Customer Wealth](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/richest_customer_wealth.rb)
     * [Two Sum](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/two_sum.rb)

data_structures/hash_table/arrays_intersection.rb

@@ -0,0 +1,56 @@
+# Given three integer arrays arr1, arr2 and arr3 sorted in strictly increasing order, return a sorted array of only the integers that appeared in all three arrays.
+#
+# Example 1:
+#
+# Input: arr1 = [1,2,3,4,5], arr2 = [1,2,5,7,9], arr3 = [1,3,4,5,8]
+# Output: [1,5]
+# Explanation: Only 1 and 5 appeared in the three arrays.
+#
+# Example 2:
+#
+# Input: arr1 = [197,418,523,876,1356], arr2 = [501,880,1593,1710,1870], arr3 = [521,682,1337,1395,1764]
+# Output: []
+#
+#
+
+#
+# Approach: Hash table
+#
+
+# Complexity Analysis
+
+# Time Complexity: O(n) - n is the total length of
+# all of the input arrays.
+# Space Complexity: O(n) - n is the total length of all of the
+# input arrays. This is because we adopted a Hash table to store all
+# numbers and their number of appearances as values.
+
+def arrays_intersection(arr1, arr2, arr3)
+  hash = Hash.new(0)
+
+  add_to_hash(arr1, hash)
+  add_to_hash(arr2, hash)
+  add_to_hash(arr3, hash)
+
+  hash.select { |key, value| value == 3 }.keys
+end
+
+def add_to_hash(arr, hash)
+  arr.count.times do |pointer|
+    value = arr[pointer]
+    hash[value] += 1
+  end
+end
+
+arr1 = [1, 2, 3, 4, 5]
+arr2 = [1, 2, 5, 7, 9]
+arr3 = [1, 3, 4, 5, 8]
+print arrays_intersection(arr1, arr2, arr3)
+# Output: [1,5]
+# Explanation: Only 1 and 5 appeared in the three arrays.
+
+arr1 = [197, 418, 523, 876, 1356]
+arr2 = [501, 880, 1593, 1710, 1870]
+arr3 = [521, 682, 1337, 1395, 1764]
+print arrays_intersection(arr1, arr2, arr3)
+# Output: []