341. Flatten Nested List Iterator展开多层数组

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:

Input: [[1,1],2,[1,1]]
Output: [1,1,2,1,1]
Explanation: By calling next repeatedly until hasNext returns false, 
             the order of elements returned by next should be: [1,1,2,1,1].

Example 2:

Input: [1,[4,[6]]]
Output: [1,4,6]
Explanation: By calling next repeatedly until hasNext returns false, 
             the order of elements returned by next should be: [1,4,6].

不是展开么，不知道为啥要用next() hasNext()
//这两个函数可能确实没啥用，只是调用一下罢了。

如果是个list，就要add list吗？也不对啊。需要再加一个recursion的函数，让它自己递归

里面用for each循环

iterator不需要初始化

public class NestedIterator implements Iterator<Integer> {
    Iterator<Integer> iterator;
    List<Integer> flattenedList = new ArrayList();

    public NestedIterator(List<NestedInteger> nestedList) {
        flatten(nestedList);
        iterator = flattenedList.iterator();
    }
    
    public void flatten(List<NestedInteger> nestedList) {
        for (NestedInteger i : nestedList) {
            if (i.isInteger()) {
                flattenedList.add(i.getInteger());
            }else {
                flatten(i.getList());
            }
        }
    }

    @Override
    public Integer next() {
        return iterator.next();
    }

    @Override
    public boolean hasNext() {
        return iterator.hasNext();
    }
}

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i = new NestedIterator(nestedList);
 * while (i.hasNext()) v[f()] = i.next();
 */