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].

本題題意是將巢狀的List展開鋪平。思路是使用一個佇列儲存鋪平後的結果，遍歷巢狀結構，如果是Integer型別，則直接加入佇列無需展開；若是List型別，則再次呼叫展開函式，將List內容展開。展開函式利用了遞迴實現。因為題目中給的介面是迭代器的相關函式，所以我們可以通過返回結果對應的iterator相關函式結果。

/**
 * // 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 LinkedList<Integer> list;
    private Iterator<Integer> iter;
    public NestedIterator(List<NestedInteger> nestedList) {
        list = new LinkedList<Integer> ();
        flatNest(nestedList);
        iter = list.iterator();
    }
    
    private void flatNest(List<NestedInteger> nestedList){
        for(NestedInteger nlist : nestedList){
            if(nlist.isInteger()){
                list.add(nlist.getInteger());
            }
            else flatNest(nlist.getList());
        }
    }

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

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

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