NestedListIterator.java

import java.util.*;

public class NestedListIterator implements Iterator<Integer> {
    interface NestedInteger {
        boolean isInteger();
        Integer getInteger();
        List<NestedInteger> getList();
    }

    Stack<Iterator<NestedInteger>> stack;
    Integer current = null;

    void updateIterator() {
        while (!stack.isEmpty() && current == null) {
            Iterator<NestedInteger> top = stack.peek();
            if (!top.hasNext()) {
                stack.pop();
                continue;
            }
            NestedInteger ni = top.next();
            if (ni.isInteger()) {
                current = ni.getInteger();
            } else {
                stack.push(ni.getList().iterator());
            }
        }
    }

    NestedListIterator(List<NestedInteger> nestedList) {
        stack = new Stack();
        stack.push(nestedList.iterator());
        updateIterator();
    }

    @Override
    public boolean hasNext() {
        return current != null ? true : false;
    }

    @Override
    public Integer next() {
        Integer temp = current;
        current = null;
        updateIterator();
        return temp;
    }
}