Leetcode 3215. Count Triplets with Even XOR Set Bits II

Given an integer array, count the number of index triplets whose bitwise XOR has an even number of 1-bits. Key insight: parity of popcount is preserved under XOR, so you can reduce the problem to counting triplets by popcount parity (all three even, or exactly two odd + one even) and compute counts combinatorially for O(n) time.