Flatten Nested Iterator

(Leetcode 341)

Question:

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:

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

Example 2:

Input: [1,[4,[6]]] Output: [1,4,6] Explanation: 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> {

    Deque<Iterator<NestedInteger>> stack = new ArrayDeque<>();
    nextInt = null;
    public NestedIterator(List<NestedInteger> nestedList) {
        Iterator<NestedInteger> it = nestedList.iterator();
        stack.push(it);
    }

    @Override
    public Integer next() {
        Integer result = nextInt.getInteger();
        nextInt = null;
        return result;
    }


    @Override
    public boolean hasNext() { 
        // keep checking the iterator and stack, until find a interger
        if(nextInt!=null) return true;
        while(!stack.isEmpty()) {
            if(stack.peek().hasNext()) {
                nextInt = stack.peek().next();
                if(nextInt.isInteger()) return true; // get integer, return
                else stack.push(nextInt.getList().iterator());// get list, push into stack and search next.
            }else stack.pop();
        }
        return false;
    }
}

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