class Solution {

    // Solution @ Sergey Leschev, Belarusian State University

    // 1512. 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.

    // 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

    // Constraints:
    // 1 <= nums.length <= 100
    // 1 <= nums[i] <= 100

    func numIdenticalPairs(_ nums: [Int]) -> Int {
        var dict = [Int: Int]()
        var count = 0

        for num in nums { dict[num] = (dict[num] ?? 0) + 1 }
        for (_, index) in dict where (index >= 2) { count += index * (index - 1) / 2 }
        return count
    }

}