Place items into earliest fitting bins

You are given k items with sizes items[0..k-1] and n bins with initial capacities caps[0..n-1]. Process items in order; for each size x, place it into the leftmost bin whose remaining capacity is at least x, then reduce that bin’s remaining capacity by x. If no such bin exists, the item remains unplaced. Return the number of unplaced items. Design an algorithm that supports up to 2×10^5 items and bins in total with O((n + k) log n) time, describing the data structure you would use.