FlattenList.java

/**
 * // 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> result = new ArrayList<Integer>();
        for (NestedInteger ele : nestedList)
            if (ele.isInteger())
                result.add(ele.getInteger());
            else
                result.addAll(flatten(ele.getList()));
        return result;
    }
}

//non-recursive version
public class Solution {

    // @param nestedList a list of NestedInteger
    // @return a list of integer
    public List<Integer> flatten(List<NestedInteger> nestedList) {
        boolean isFlat = true;
        List<NestedInteger> ls = nestedList;
        while (isFlat) {
            isFlat = false;
            List<NestedInteger> newLs = new ArrayList<>();
            for (NestedInteger ni : ls) {
                if (ni.isInteger()) {
                    newLs.add(ni);
                } else {
                    newLs.addAll(ni.getList());
                    isFlat = true;
                }
            }
            ls = newLs;
        }
        List<Integer> r = new ArrayList<>();
        for (NestedInteger ni : ls) {
            r.add(ni.getInteger());
        }
        return r;
    }
}