Leetcode 3587. Minimum Adjacent Swaps to Alternate Parity

Given an array of distinct integers, find the minimum number of adjacent swaps to reorder it so parities alternate (even/odd/even/...), or return -1 if impossible. Key insight: feasibility requires the counts of evens and odds to differ by at most one, and the min swaps equals the minimal total displacement when matching current indices of one parity to target positions (try both starting-parity patterns).