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,

numEquivDominoPairs.js

/**
 * Counts the number of equivalent domino pairs.
 * Two dominoes are equivalent if they have the same numbers, regardless of order.
 * 
 * @param {number[][]} dominoes - A list of domino pairs represented as 2-element arrays.
 * @return {number} - The total number of equivalent domino pairs.
 */
var numEquivDominoPairs = function(dominoes) {
    // If there are no dominoes, return 0
    if (!dominoes || dominoes.length === 0) {
        return 0;
    }

    let numPairs = 0; // Counter for equivalent pairs
    let pairCountMap = new Map(); // Stores occurrences of normalized domino pairs

    for (let [a, b] of dominoes) {
        // Ensure the smaller number comes first to standardize the representation
        const key = a < b ? `${a},${b}` : `${b},${a}`;

        // Retrieve the current count of this pair (default to 0 if not found)
        const currentCount = pairCountMap.get(key) || 0;

        // Each new occurrence of the pair can form `currentCount` additional pairs
        numPairs += currentCount;

        // Update the pair count in the map
        pairCountMap.set(key, currentCount + 1);
    }

    return numPairs;
};