Sort Even and Odd Indices Independently

Treat positions, not number parity, as the grouping rule. With zero-based indexing, collect values at indices 0,2,4,... into one list and values at 1,3,5,... into another. Sort the even-index list ascending and the odd-index list descending, then write them back by walking the original array: an even position consumes the next ascending value, and an odd position consumes the next descending value. This direct decomposition is clearer than trying to encode two ordering rules in one comparator. Be explicit about zero-based indexing, because one-based wording reverses which physical positions are called even. If mutation is forbidden, create a result array instead. Collection and reconstruction are O(n); sorting dominates at O(n log n) time; the straightforward version uses O(n) extra space. Empty and single-element arrays need no special-case code beyond normal loops.