LeetCode 1128. 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

找到相同组合的个数，2,3 跟3,2是一样的，所以我们就用大小顺序放在一起组成字符串，作为hashmap的key，放进去，最后一次遍历value>1 的情况，即可；

下面是代码：

Runtime: 24 ms, faster than 39.74% of Java online submissions for Number of Equivalent Domino Pairs.

Memory Usage: 52.4 MB, less than 38.03% of Java online submissions for Number of Equivalent Domino Pairs.