22. 平面列表

给定一个列表，该列表中的每个要素要么是个列表，要么是整数。将其变成一个只包含整数的简单列表。

样例

给定 [1,2,[1,2]]，返回 [1,2,1,2]。

给定 [4,[3,[2,[1]]]]，返回 [4,3,2,1]。

/**
 * // 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 getList();
 * }
 */
public class Solution {

    // @param nestedList a list of NestedInteger
    // @return a list of integer
    public List flatten(List nestedList) {
        // Write your code here
        List list = new ArrayList<>();
		doFlattern(nestedList, list);
		return list;
	}
	
	public void doFlattern(List nestedList, List list) {
		if (nestedList != null) {
			for (int i = 0; i < nestedList.size(); i++) {
				if (nestedList.get(i).isInteger()) {
					list.add(nestedList.get(i).getInteger());
				} else {
					doFlattern(nestedList.get(i).getList(), list);
				}
			}
		}
	}
    
}