Problem

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

Pre analysis

Will keep track of previous occurence of dominoes and will keep track of count of occurence of each dominoes.