Number of Equivalent Domino Pairs

Given a list of dominoes, dominoes[i] = [a, b] is equivalent to dominoes[j] = [c, d] if and only if either (a == c and b == d), or (a == d and b == c) - that is, one domino can be rotated to be equal to another domino.

Return the number of pairs (i, j) for which 0 <= i < j < dominoes.length, and dominoes[i] is equivalent to dominoes[j].

Example 1:

Input: dominoes = [[1,2],[2,1],[3,4],[5,6]]
Output: 1

Example 2:

Input: dominoes = [[1,2],[1,2],[1,1],[1,2],[2,2]]
Output: 3

Constraints:

• 1 <= dominoes.length <= 4 * 104
• dominoes[i].length == 2
• 1 <= dominoes[i][j] <= 9

SOLUTION:

class Solution:
    def numEquivDominoPairs(self, dominoes: List[List[int]]) -> int:
        n = len(dominoes)
        ctr = {}
        for i in range(n):
            curr = tuple(sorted(dominoes[i]))
            ctr[curr] = ctr.get(curr, 0) + 1
        op = 0
        for pair in ctr:
            op += ctr[pair] * (ctr[pair] - 1) // 2
        return op