Flatten List

Problem

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

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].

Code - Java

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * public interface NestedInteger {
 * <p>
 * // @return true if this NestedInteger holds a single integer,
 * // rather than a nested list.
 * public boolean isInteger();
 * <p>
 * // @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();
 * <p>
 * // @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> result = new ArrayList<>();

        for (NestedInteger i : nestedList) {
            if (i.isInteger()) {
                result.add(i.getInteger());
            } else {
                result.addAll(flatten(i.getList()));
            }
        }
        return result;
    }
}