Flatten List(平面列表)

问题

Given a list, each element in the list can be a list or integer. flatten it into a simply list with integers.

Notice

If the element in the given list is a list, it can contain list too.

Have you met this question in a real interview? Yes
Example
Given [1,2,[1,2]], return [1,2,1,2].

Given [4,[3,[2,[1]]]], return [4,3,2,1].

分析

使用递归来简化操作。

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 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 res=new ArrayList();
        tree(res,nestedList);
        return res;
    }
    private void tree(List res,List nestedList){
        if(nestedList==null||nestedList.isEmpty()){
            return;
        }
        for(NestedInteger nest:nestedList){
            if(nest.isInteger()){
                res.add(nest.getInteger());
            }else{
                tree(res,nest.getList());
            }
        }
    }
}