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 * // 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  {

    // @param nestedList a list of NestedInteger
    // @return a list of integer
    public List<Integer> flatten(List<NestedInteger> nestedList) {
        // Write your code here
        List<Integer> res=new ArrayList<>();
        res=tree(res,nestedList);
        return res;
        
    }
    
    public List<Integer> tree(List<Integer> res,List<NestedInteger> nestedList)
    {
        //if(nestedList==null||nestedList.isEmpty())
        //return null;
        for(NestedInteger item:nestedList)
        {
            if(item.isInteger())
            {
                res.add(item.getInteger());
            }
            else
            {
                tree(res,item.getList());
            }
        }
        return res;
    }
}