A question before this is the Nested List Weight Sum, and it requires recursion to solve. As it carries to this problem that we will need recursion to solve it. But since we need to access each NestedInteger at a time, we will use a stack to help.

這題我做的時候就覺得要是能用遞歸就好了，但是由于一次只能next()取一個，所以不知咋辦了?？戳艘谎踫olutions，用了stack，然后去健身了，健身的時候想到了，就是把get到的東西不停的往stack里push，剛才自己試著實現(xiàn)了一下，代碼基本跟答案一致，但是我一開始想的是把hasNext里面的操作放到next()里面，但是那樣有個問題，如果是[[]]這種情況，我最終返回了一個null，它結(jié)果是[null]，而需要的是[]。

所以，堆棧操作要放在hashNext里。

    Stack<NestedInteger> stack = new Stack<>();

    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()) {
            //不可以直接pop
            NestedInteger ni = stack.peek();
            if (ni.isInteger()) {
                return true;
            }
            stack.pop();
            List<NestedInteger> list = ni.getList();
            for (int i = list.size() - 1; i >= 0; i--) {
                stack.push(list.get(i));
            }
        }
        return false;
    }

有空做一下Nested List Weight Sum這題。
今天要熬夜做ppt。