Flatten Nested List Iterator

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 callingnextrepeatedly untilhasNextreturns false, the order of elements returned bynextshould be:[1,1,2,1,1].

Example 2:
Given the list[1,[4,[6]]],

By callingnextrepeatedly untilhasNextreturns false, the order of elements returned bynextshould be:[1,4,6].

题意：压平嵌套链表的迭代器

代码

/**
 * // 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> {

    private Stack<NestedInteger> stack;

    public NestedIterator(List<NestedInteger> nestedList) {
        stack = new Stack<>();
        pushListElements(nestedList);
    }

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

    @Override
    public boolean hasNext() {
        if (stack.isEmpty()) {
            return false;
        }

        while (!stack.peek().isInteger()) {
            pushListElements(stack.pop().getList());
            if (stack.isEmpty()) {
                return false;
            }
        }
        return true;
    }

    private void pushListElements(List<NestedInteger> list) {
        for (int i = list.size() - 1; i >= 0; i--) {
            stack.push(list.get(i));
        }
    }
}

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