flatten-nested-list-iterator.java

// Problem link - https://leetcode.com/problems/flatten-nested-list-iterator/

/**
 * // 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 empty list if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
public class NestedIterator implements Iterator<Integer> {
    private Stack<ListIterator<NestedInteger>> lists;
    int index;
    public NestedIterator(List<NestedInteger> nestedList) {
        lists =  new Stack<>();
        lists.push(nestedList.listIterator());
    }

    @Override
    public Integer next() {
        hasNext();
        return lists.peek().next().getInteger();
    }

    @Override
    public boolean hasNext() {
        while(!lists.empty()){
            if(!lists.peek().hasNext()){
                lists.pop();
            } else {
                NestedInteger x = lists.peek().next();
                if(x.isInteger()){
                    return lists.peek().previous() == x;
                }
                lists.push(x.getList().listIterator());
            }
        }

        return false;
    }
}

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