Medium

看的答案，我自己用stack的题还是做少了，特别不熟。
关键点：

• 整个数据结构由一个Stack支撑
• 一开始把所有是Integer 或者nested list的所有NestedInteger都加入到stack里面去
• 根绝题意，iterate出来的都得是Integer, 所以我们hasNext()的关键就是要保证我们下一个pop()出来的必须得是Integer, 如果不行那就返回false. 所以我们要每次peek()一下现在stack里面的元素，看看是不是Integer. 如果不是，还得继续Flatten. Flatten的过程就是把它pop()出来，然后逆序放入stack里。画上几笔，就可以蛮清楚地看到it works.
• next()就直接返回stack.peek().getInteger()就好了

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * public interface NestedInteger {
 *
 *     // @return true if this NestedInteger holds a single integer, rather than a nested list.
 *     public boolean isInteger();
 *
 *     // @return the single integer that this NestedInteger holds, if it holds a single integer
 *     // Return null if this NestedInteger holds a nested list
 *     public Integer getInteger();
 *
 *     // @return the nested list that this NestedInteger holds, if it holds a nested list
 *     // Return null if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */

public class NestedIterator implements Iterator<Integer> {
    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()){
            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;
    }
}

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