Description

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.

Example

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

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

Solution

/**
 * // 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 Solution {

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