Formatted question description: https://leetcode.ca/all/1128.html

1128. Number of Equivalent Domino Pairs (Easy)

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

 

Constraints:

• 1 <= dominoes.length <= 40000
• 1 <= dominoes[i][j] <= 9

Related Topics:
Array

Solution 1.

// OJ: https://leetcode.com/problems/number-of-equivalent-domino-pairs/

// Time: O(NlogN)
// Space: O(N)
class Solution {
public:
    int numEquivDominoPairs(vector<vector<int>>& A) {
        map<vector<int>, int> m;
        int ans = 0;
        for (auto &v : A) {
            sort(begin(v), end(v));
            ans += m[v];
            m[v]++;
        }
        return ans;
    }
};

Solution 2.

// OJ: https://leetcode.com/problems/number-of-equivalent-domino-pairs/

// Time: O(NlogN)
// Space: O(1)
class Solution {
public:
    int numEquivDominoPairs(vector<vector<int>>& A) {
        for (auto &v : A) sort(begin(v), end(v));
        sort(begin(A), end(A));
        int ans = 0;
        for (int i = 0, N = A.size(); i < N;) {
            int start = i;
            while (i < N && A[i] == A[start]) ++i;
            int len = i - start;
            ans += (len - 1) * len / 2;
        }
        return ans;
    }
};

Solution 3.

// OJ: https://leetcode.com/problems/number-of-equivalent-domino-pairs/

// Time: O(N)
// Space: O(1)
class Solution {
public:
    int numEquivDominoPairs(vector<vector<int>>& A) {
        int cnt[10][10] = {}, ans = 0;
        for (auto &v : A) {
            if (v[0] > v[1]) swap(v[0], v[1]);
            ans += cnt[v[0]][v[1]]++;
        }
        return ans;
    }
};

Java

class Solution {
    public int numEquivDominoPairs(int[][] dominoes) {
        int length = dominoes.length;
        for (int i = 0; i < length; i++) {
            int[] domino = dominoes[i];
            if (domino[0] > domino[1]) {
                int temp = domino[0];
                dominoes[i][0] = dominoes[i][1];
                dominoes[i][1] = temp;
            }
        }
        Arrays.sort(dominoes, new Comparator<int[]>() {
            public int compare(int[] array1, int[] array2) {
                if (array1[0] != array2[0])
                    return array1[0] - array2[0];
                else
                    return array1[1] - array2[1];
            }
        });
        int equivalentCount = 0;
        int consecutive = 1;
        for (int i = 1; i < length; i++) {
            int[] prevDomino = dominoes[i - 1];
            int[] curDomino = dominoes[i];
            if (prevDomino[0] == curDomino[0] && prevDomino[1] == curDomino[1])
                consecutive++;
            else {
                equivalentCount += consecutive * (consecutive - 1) / 2;
                consecutive = 1;
            }
        }
        if (consecutive > 1)
            equivalentCount += consecutive * (consecutive - 1) / 2;
        return equivalentCount;
    }
}

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