1128. Number of Equivalent Domino Pairs

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 * 10^4

• dominoes[i].length == 2

• 1 <= dominoes[i][j] <= 9

Solution (Java)

class Solution {
    public int numEquivDominoPairs(int[][] dominoes) {
        Map<Integer, Integer> map = new HashMap<>();
        int count = 0;
        for (int[] dominoe : dominoes) {
            int smaller = Math.min(dominoe[0], dominoe[1]);
            int bigger = Math.max(dominoe[0], dominoe[1]);
            int key = smaller * 10 + bigger;
            count += map.getOrDefault(key, 0);
            map.put(key, map.getOrDefault(key, 0) + 1);
        }
        return count;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).