Twice-As-Big Pairs — Solution & Editorial

Back to the Problem

Merge-sort counting, like inversions, but the crossing condition a_i > 2a_j needs its own two-pointer pass before merging (a BIT over compressed 2·values works too).

Complexity: O(N log N)

Watch out: Negative values: 2·a_j with a_j negative flips intuition — the comparison must be exact, not index tricks.