Flatten Nested List Iterator

Problem

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> {

    Stack<NestedInteger> stack = new Stack<>();

    public NestedIterator(List<NestedInteger> nestedList) {
        for (int i = nestedList.size() - 1; i >= 0; i--) stack.push(nestedList.get(i));
    }

    @Override
    public Integer next() {
        return stack.pop().getInteger();
    }

    //We modify the stack here
    @Override
    public boolean hasNext() {
       while (!stack.isEmpty()) {
            NestedInteger cur = stack.peek();
            if (cur.isInteger()) return true;
            stack.pop();
            List<NestedInteger> list = cur.getList();
            for (int i = list.size() - 1; i >= 0; i--) stack.push(list.get(i));
       }
        return false;
    }
}

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

Analysis

We have to use Stack in this problem This is because when we meet a list, we have to uncover the list and push each integer at the beginning, to make next() correct The problem guarantees that hasNext() will get called before next(), hence we modify the stack() in hasNext() method