Merge pull request #64 from vbrazo/add-first-array-example

Array data structure: Get products of all other elements

DIRECTORY.md

@@ -3,6 +3,8 @@
   * [Merkle Hellman Cryptosystem](https://github.com/TheAlgorithms/Ruby/blob/master/ciphers/merkle_hellman_cryptosystem.rb)
 
 ## Data Structures
+  * Arrays
+    * [Get products of all other elements in Ruby](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/get_products_of_all_other_elements.rb)
   * Binary Trees
     * [Inorder Traversal](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/binary_trees/inorder_traversal.rb)
     * [Invert](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/binary_trees/invert.rb)

data_structures/arrays/get_products_of_all_other_elements.rb

@@ -0,0 +1,89 @@
+# Arrays - Get Products of all other elements in Ruby
+
+# Algorithm challenge description:
+# Given an array of integers, return a new array such that
+# each element at index `i` of the new array is the product of
+# all the numbers in the original array except the one at `i`.
+
+#
+# 1. Brute force solution
+#
+def calculate_products_of_all_other_elements(nums)
+  product_of_other_elements = Array.new(nums.length, 1)
+
+  nums.each_with_index do |num1, i|
+    nums.each_with_index do |num2, j|
+      if (i != j)
+        product_of_other_elements[i] = product_of_other_elements[i] * num2
+      end
+    end
+  end
+
+  product_of_other_elements
+end
+
+puts(calculate_products_of_all_other_elements([1, 2, 3]))
+
+#
+# 2. O(n) time and space
+# Arrays - Get Products of all other elements in Ruby
+#
+
+# Generates prefix products
+def build_prefix_products(nums)
+  prefix_products = []
+
+  nums.each do |num|
+    if prefix_products.count > 0
+      prefix_products << (prefix_products.last * num)
+    else
+      prefix_products << num
+    end
+  end
+
+  return prefix_products
+end
+
+# Generates suffix products
+def build_suffix_products(nums)
+  suffix_products = []
+
+  nums.reverse.each do |num|
+    if suffix_products.count > 0
+      suffix_products << (suffix_products.last * num)
+    else
+      suffix_products << num
+    end
+  end
+
+  return suffix_products
+end
+
+# Builds output
+def output(prefix_products, suffix_products, nums)
+  result = []
+
+  nums.reverse.each_with_index do |num, index|
+    if index == 0
+      result << suffix_products[index + 1]
+    elsif index == nums.length - 1
+      result << prefix_products[index - 1]
+    else
+      result << (prefix_products[index - 1] * suffix_products[index + 1])
+    end
+  end
+
+  return result
+end
+
+# Generate result from the product of prefixes and suffixes
+def products(nums)
+  prefix_products = build_prefix_products(nums)
+  suffix_products = build_suffix_products(nums)
+  suffix_products = suffix_products.reverse
+
+  return output(prefix_products, suffix_products, nums)
+end
+
+puts(products([1, 2, 3]))
+# => [6, 3, 2]