Flatten Nested List Iterator

Use a stack of nested items instead of recursively flattening everything at construction. Initialize it with the top-level list in reverse order so the first logical item is on top. In hasNext, while the stack is nonempty and its top is a list, pop that list and push its children in reverse order. When the loop finishes, the top is either an integer or the stack is empty. next calls hasNext, then pops and returns the integer. Keeping normalization in hasNext makes repeated hasNext calls safe and means next never needs to understand list nesting. Test empty lists at multiple depths, a single integer, deeply nested lists, and alternating empty and nonempty lists. Each item is pushed and popped at most once, so total iteration work is O(N) for N nested items, amortized O(1) per next, and stack space is O(depth plus pending siblings).