#1128 - Number of Equivalent Domino Pairs

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Problem 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].

Solution

/**
 * @param {number[][]} dominoes
 * @return {number}
 */
var numEquivDominoPairs = function(dominoes) {
  const map = new Map();
  let result = 0;

  for (const [a, b] of dominoes) {
    const key = Math.min(a, b) * 10 + Math.max(a, b);
    const count = map.get(key) || 0;
    result += count;
    map.set(key, count + 1);
  }

  return result;
};