Number of Equivalent Domino Pairs — LeetCode 1128 Python Solution

The 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

Input

dominoes = [[1,2],[2,1],[3,4],[5,6]]

Output

1

Python solution

class Solution:
    def numEquivDominoPairs(self, dominoes: List[List[int]]) -> int:
        cnt = Counter()
        ans = 0
        for a, b in dominoes:
            x = a * 10 + b if a < b else b * 10 + a
            ans += cnt[x]
            cnt[x] += 1
        return ans

Complexity

Pattern: Hash Map

Trade memory for time: remember what you have seen so the second pass never happens. LeetCode 1128. Number of Equivalent Domino Pairs is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Hash Table and Counting.

The hash map guide has the Python template for the pattern and the 709 LeetCode problems that use it.

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