Add hash approach

data_structures/arrays/good_pairs.rb

@@ -37,6 +37,39 @@ puts(num_identical_pairs(nums))
 # Output: 6
 # Explanation: Each pair in the array are good.
 
+nums = [1, 2, 3]
+puts(num_identical_pairs(nums))
+# Output: 0
+
+#
+# Approach 2: Hash
+#
+def num_identical_pairs(nums)
+  hash = Hash.new(0)
+
+  nums.each do |num|
+    hash[num] = hash[num] + 1
+  end
+
+  counter = 0
+  # Count how many times each number appears.
+  # If a number appears n times, then n * (n – 1) / 2 good pairs
+  # can be made with this number.
+  hash.values.each do |val|
+    counter += (val * (val - 1) / 2)
+  end
+
+  counter
+end
+
+nums = [1, 2, 3, 1, 1, 3]
+puts(num_identical_pairs(nums))
+# Output: 4
+
+nums = [1, 1, 1, 1]
+puts(num_identical_pairs(nums))
+# Output: 6
+
 nums = [1, 2, 3]
 puts(num_identical_pairs(nums))
 # Output: 0