LeetCode - Flatten Nested List Iterator

FlattenNestedListIterator.java

// 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].

/**
 * // 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> {
    
    Iterator<NestedInteger> iterator;
    Stack<Iterator> stack = new Stack<Iterator>();
    Integer cur = null;
    
    public NestedIterator(List<NestedInteger> nestedList) {
        this.iterator = nestedList.iterator();
    }

    @Override
    public Integer next(){
        return cur;
    }

    @Override
    public boolean hasNext() {
        while(iterator.hasNext() || !stack.isEmpty()){
            if(iterator.hasNext()){
                NestedInteger nestedInteger = iterator.next();
                if(nestedInteger.isInteger()){
                   cur = nestedInteger.getInteger();
                   return true;
                } else {
                    if(iterator.hasNext()) stack.push(iterator);
                    iterator = nestedInteger.getList().iterator();
                }
            } else{
                iterator = 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();
 */