Add hash solution

data_structures/arrays/intersection.rb

@@ -38,6 +38,38 @@ nums2 = [2, 2]
 puts intersect(nums1, nums2)
 # => [2,2]
 
+nums1 = [4, 9, 5]
+nums2 = [9, 4, 9, 8, 4]
+puts intersect(nums1, nums2)
+# => [4,9]
+
+#
+# Approach 2: Hash
+#
+# Time Complexity: O(n)
+#
+def intersect(arr1, arr2)
+  result = []
+
+  hash = Hash.new(0)
+
+  arr2.each {|num| hash[num] += 1 }
+
+  arr1.each do |num|
+    if hash.has_key?(num)
+      result << num if hash[num] >= 1
+      hash[num] -= 1
+    end
+  end
+  
+  result
+end
+
+nums1 = [1, 2, 2, 1]
+nums2 = [2, 2]
+puts intersect(nums1, nums2)
+# => [2,2]
+
 nums1 = [4, 9, 5]
 nums2 = [9, 4, 9, 8, 4]
 puts intersect(nums1, nums2)