Leetcode 1128. Number of Equivalent Domino Pairs

Count how many index pairs (i < j) of dominoes are equivalent up to rotation (i.e., [a,b] equals [c,d] if {a,b} = {c,d}). Normalize each domino to an order-insensitive form (e.g., (min,max)) and total the combinations from duplicate normalized forms; n ≤ 4×10^4 and values are 1..9.