等价多米诺骨牌对的数量

/*
 * Problem: Given a list of dominoes, find the number of pairs (i, j) such that dominoes[i] and dominoes[j] are equivalent.
 * Two dominoes [a, b] and [c, d] are equivalent if and only if (a == c and b == d) or (a == d and b == c).
 * 
 * Approach:
 * 1. Use Java Streams to count the frequency of each domino configuration. Normalize each domino to a consistent form (min, max).
 * 2. For each domino configuration, if the count is more than 1, calculate the number of valid pairs using the combination formula n * (n - 1) / 2.
 */
import java.util.*;
import java.util.stream.Collectors;

public class DominoPairsStream {
    public int numEquivDominoPairs(int[][] dominoes) {
        Map<String, Long> map = Arrays.stream(dominoes)
                .map(domino -> {
                    int a = domino[0];
                    int b = domino[1];
                    return a < b ? a + "," + b : b + "," + a;
                })
                .collect(Collectors.groupingBy(e -> e, Collectors.counting()));

        return map.values().stream()
                .filter(val -> val > 1)
                .mapToInt(val -> (int) (val * (val - 1) / 2))
                .sum();
    }
}