LeetCode 1128: Number of Equivalent Domino Pairs Solution

Master LeetCode problem 1128 (Number of Equivalent Domino Pairs), a easy challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

1128. Number of Equivalent Domino Pairs

Easy

Array Hash Table Counting

Problem Explanation

Explanation

To solve this problem, we can iterate through the given list of dominoes and convert each domino to a canonical representation by ensuring that the smaller number is always in the first position. We can then use a hashmap to store the count of each canonical representation. For each canonical representation encountered, we can calculate the number of pairs that can be formed using the count of that representation. Finally, we sum up the total pairs across all representations.

Time complexity: O(n), where n is the number of dominoes.
Space complexity: O(n) to store the counts of the canonical representations.

Solution Code

class Solution {
    public int numEquivDominoPairs(int[][] dominoes) {
        Map<Integer, Integer> countMap = new HashMap<>();
        int pairs = 0;
        
        for (int[] domino : dominoes) {
            int val = Math.min(domino[0], domino[1]) * 10 + Math.max(domino[0], domino[1]);
            pairs += countMap.getOrDefault(val, 0);
            countMap.put(val, countMap.getOrDefault(val, 0) + 1);
        }
        
        return pairs;
    }
}

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