README_EN.md

1128. Number of Equivalent Domino Pairs

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Description

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

Solutions

Python3

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

Java

class Solution {
    public int numEquivDominoPairs(int[][] dominoes) {
        int ans = 0;
        int[] counter = new int[100];
        for (int[] d : dominoes) {
            int v = d[0] > d[1] ? d[0] * 10 + d[1] : d[1] * 10 + d[0];
            ans += counter[v];
            ++counter[v];
        }
        return ans;
    }
}

class Solution {
    public int numEquivDominoPairs(int[][] dominoes) {
        int[] counter = new int[100];
        for (int[] d : dominoes) {
            int v = d[0] > d[1] ? d[0] * 10 + d[1] : d[1] * 10 + d[0];
            ++counter[v];
        }
        int ans = 0;
        for (int c : counter) {
            ans += c * (c - 1) / 2;
        }
        return ans;
    }
}

C++

class Solution {
public:
    int numEquivDominoPairs(vector<vector<int>>& dominoes) {
        vector<int> counter(100);
        int ans = 0;
        for (auto& d : dominoes)
        {
            int v = d[0] > d[1] ? d[0] * 10 + d[1] : d[1] * 10 + d[0];
            ans += counter[v];
            ++counter[v];
        }
        return ans;
    }
};

Go

func numEquivDominoPairs(dominoes [][]int) int {
	counter := make([]int, 100)
	for _, d := range dominoes {
		if d[1] < d[0] {
			d[0], d[1] = d[1], d[0]
		}
		v := d[0]*10 + d[1]
		counter[v]++
	}
	ans := 0
	for _, c := range counter {
		ans += c * (c - 1) / 2
	}
	return ans
}

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