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

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

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

 题目含义：给定一个嵌套的整数列表，实现一个迭代器将其展开。每一个元素或者是一个整数，或者是一个列表 -- 其元素也是一个整数或者其他列表。

/**
 * // 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 List<Integer> flattenedList;
    private Iterator<Integer> it;

    public NestedIterator(List<NestedInteger> nestedList) {
        flattenedList = new LinkedList<Integer>();
        flatten(nestedList);
        it = flattenedList.iterator();
    }
    private 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 it.next();
    }

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

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