LeetCode： Number of Good Pairs

Question

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.

Example 1:

Input: nums = [1,2,3,1,1,3]

Output: 4

Explanation: There are 4 good pairs (0,3), (0,4), (3,4), (2,5) 0-indexed.

Example 2:

Input: nums = [1,1,1,1]

Output: 6

Explanation: Each pair in the array are good.

Example 3:

Input: nums = [1,2,3]

Output: 0

Solution#1

def num_identical_pairs(nums)
  dict = Hash.new(0)
  nums.each { |num| dict[num] += 1}
  dict.values.select{|num| num > 1}.map{ |num| ((num-1)*num)/2}.sum
end

Solution#2

def num_identical_pairs(nums)
  counter = 0
  nums.each_with_index do |first_number, index|
    nums[index+1..-1].each do |second_number|
      counter += 1 if first_number == second_number
    end
  end
  counter
end