1128. Number of Equivalent Domino Pairs

Problem Link

Description

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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

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Python3

class Solution:
    def numEquivDominoPairs(self, dominoes: List[List[int]]) -> int:
        pair = defaultdict(int)
        res = 0
 
        for a, b in dominoes:
            if a < b:
                res += pair[(a, b)]
                pair[(a, b)] += 1
            else:
                res += pair[(b, a)]
                pair[(b, a)] += 1
            
        return res