Given a nested list of integers, implement an iterator to flatten it.
Each element is either an integer, or a list -- whose elements may also be integers or other lists.

Example 1:
Given the list [[1,1],2,[1,1]],
By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].

Example 2:
Given the list [1,[4,[6]]],
By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].

Solution1：Stack

Time Complexity: O(N) Space Complexity: O(N)

Solution2：Stack Round1

Time Complexity: O(N) Space Complexity: O(N)

Solution1 Code：

public class NestedIterator implements Iterator<Integer> {

    Deque<NestedInteger> stack = new ArrayDeque<>();
    public NestedIterator(List<NestedInteger> nestedList) {
        for(int i = nestedList.size() - 1; i >= 0; i--) {
            stack.push(nestedList.get(i));
        }
    }


    @Override
    public Integer next() {
        return stack.pop().getInteger();
    }

    @Override
    public boolean hasNext() {
        while(!stack.isEmpty()) {
            NestedInteger curr = stack.peek();
            if(curr.isInteger()) {
                return true;
            }
            stack.pop();
            for(int i = curr.getList().size() - 1; i >= 0; i--) {
                stack.push(curr.getList().get(i));
            }
        }
        return false;
    }
}

Solution2 Round1 Code：

public class NestedIterator implements Iterator<Integer> {

    private Deque<NestedInteger> stack;
    
    public NestedIterator(List<NestedInteger> nestedList) {
        stack = new ArrayDeque<>();
        for(int i = nestedList.size() - 1; i >= 0; i--) {
            stack.push(nestedList.get(i));
        } 
    }

    @Override
    public Integer next() {
        return stack.pop().getInteger();
    }

    @Override
    public boolean hasNext() {
        while(stack.size() > 0) {
            NestedInteger elem = stack.peek();
            if(elem.isInteger()) {
                return true;
            }
            else {
                elem = stack.pop();
                List<NestedInteger> list = elem.getList();
                for(int i = list.size() - 1; i >= 0; i--) {
                    stack.push(list.get(i));
                } 
            }
        }
        return false; 
    }
}