Merge pull request #148 from jsca-kwok/jk-good-pairs

Good pairs algorithm

DIRECTORY.md

@@ -25,6 +25,7 @@
     * [Find The Highest Altitude](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/find_the_highest_altitude.rb)
     * [Fizz Buzz](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/fizz_buzz.rb)
     * [Get Products Of All Other Elements](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/get_products_of_all_other_elements.rb)
+    * [Good Pairs](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/good_pairs.rb)
     * [Intersection](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/intersection.rb)
     * [Next Greater Element](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/next_greater_element.rb)
     * [Remove Elements](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/remove_elements.rb)
@@ -51,6 +52,7 @@
     * [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)
+    * [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)
     * [Two Sum](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/hash_table/two_sum.rb)
   * Linked Lists

data_structures/arrays/good_pairs.rb

@@ -0,0 +1,42 @@
+# Challenge name: Number of good pairs
+#
+# Given an array of integers nums.
+# A pair (i,j) is called good if nums[i] == nums[j] and i < j.
+# Return the number of good pairs.
+#
+# @param {Integer[]} nums
+# @return {Integer}
+#
+
+#
+# Approach 1: Brute Force
+#
+# Time Complexity: O(n^2)
+#
+def num_identical_pairs(nums)
+  count = 0
+  nums.each_with_index do |num, i|
+    target = num
+    nums.each_with_index do |num, j|
+      next if i >= j
+      if num == target
+        count += 1
+      end
+    end
+  end
+  count
+end
+
+nums = [1, 2, 3, 1, 1, 3]
+puts(num_identical_pairs(nums))
+# Output: 4
+# Explanation: There are 4 good pairs (0,3), (0,4), (3,4), (2,5) 0-indexed.
+
+nums = [1, 1, 1, 1]
+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

data_structures/hash_table/good_pairs.rb

@@ -0,0 +1,43 @@
+# Challenge name: Number of good pairs
+#
+# Given an array of integers nums.
+# A pair (i,j) is called good if nums[i] == nums[j] and i < j.
+# Return the number of good pairs.
+#
+# @param {Integer[]} nums
+# @return {Integer}
+#
+
+#
+# Approach 1: Hash
+#
+# Time Complexity: O(n)
+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