Flatten Nested List Iterator

Given a nested list of integers, implement an iterator to flatten it.

Each element is either an integer, or a list -- whose elements may also be integers or other lists.

Example 1: Given the list [[1,1],2,[1,1]],

By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].

Example 2: Given the list [1,[4,[6]]],

By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].

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 NestedIterator implements Iterator<Integer> {

    private List<Integer> nums;
    private Iterator<Integer> iterator;

    public NestedIterator(List<NestedInteger> nestedList) {
        nums = new ArrayList<>();
        for(NestedInteger nestedInteger: nestedList) {
            add(nestedInteger);
        }
        iterator = nums.iterator();
    }

    private void add(NestedInteger nestedInteger) {
        if (nestedInteger.isInteger()) {
            nums.add(nestedInteger.getInteger());
        } else {
            for(NestedInteger nestedInteger1: nestedInteger.getList()) {
                add(nestedInteger1);
            }
        }
    }

    public Integer next() {
        return iterator.next();
    }

    public boolean hasNext() {
        return iterator.hasNext();
    }
}

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i = new NestedIterator(nestedList);
 * while (i.hasNext()) v[f()] = i.next();
 */