251. Flatten 2D Vector

https://leetcode.com/problems/flatten-2d-vector/description/

题目很简单，两个思路：

• 双 index
• 双 iterator

public class Vector2D implements Iterator<Integer> {
    private Iterator<List<Integer>> listsIter;
    private Iterator<Integer> listIter;
    
    public Vector2D(List<List<Integer>> vec2d) {
        listsIter = vec2d.iterator();
        if (listsIter.hasNext()) {
            listIter = listsIter.next().iterator();
        }
    }

    @Override
    public Integer next() {
        // hasNext() should always be called before next(), so these few lines are not neccessary.
        // while (!listIter.hasNext()) {
        //     listIter = listsIter.next().iterator();
        // }
        
        return listIter.next();
    }

    @Override
    public boolean hasNext() {
        if (listIter == null) {
            return false;
        }
        
        while (listsIter.hasNext() && !listIter.hasNext()) {
            listIter = listsIter.next().iterator();
        }
        
        return listIter.hasNext();
    }
}

Solution from: https://www.youtube.com/watch?v=Hwk1CD86YmA&list=PLvyIyKZVcfAn_9zI9qcHTtNbhLz6OFd_c&index=3

List get 复杂度取决于 Class。ArrayList O(1), LinkedList O(n)