Number of Equivalent Domino Pairs in Python

Suppose we have a list of dominos. Each domino has two numbers. Two dominos D[i] = [a, b] and D[j] = [c, d] will be same if a = c and b = d, or a = d and b = c. So one domino can be reversed. We have to return number of pairs (i, j) for which 0 <= i < j < length of Dominos, and determine D[i] is equivalent to D[j]. So if the domino list is like [[1, 2], [2, 1], [3, 4], [6, 5]]. The output will be 1

To solve this, we will follow these steps −

• let answer = 0
• for each pair p in dominos list −
  • sort pair p
  • Then store the frequency of each domino into D
• for b in values in D −
  • answer := answer + (b * (b - 1))/2
• return answer

Example

Let us see the following implementation to get better understanding −

 Live Demo

class Solution(object):
   def numEquivDominoPairs(self, dominoes):
      d = {}
      ans = 0
      for i in dominoes:
         i.sort()
         i = tuple(i)
         if i not in d:
            d[i]= 1
         else:
            d[i]+=1
      for b in d.values():
         ans += ((b*(b-1))//2)
      return ans
ob1 = Solution()
print(ob1.numEquivDominoPairs([[1,2],[2,1],[3,4],[5,6], [4,3]]))

Input

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

Output

2