Design a nested-list iterator

You are given a nested structure that can contain integers or further lists. Design an iterator over this structure that returns integers in left-to-right order without pre-flattening the entire input. Implement a class NestedIterator with: constructor(nestedList), boolean hasNext(), and int next(). The iterator must use O(d) extra space where d is the maximum nesting depth, and hasNext()/next() should be amortized O( 1). Avoid recursion by using an explicit stack of iterators or indexes. Additionally, implement a recursive pre-flattening approach, compare both methods in time and space complexity, and discuss edge cases such as empty lists, deeply nested empty lists, and integer bounds.