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].

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/**
* // 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() {
hasNext();
return stack.pop().getInteger();
}

@Override
public boolean hasNext() {
while (!stack.isEmpty()) {
NestedInteger cur = stack.peek();
if (cur.isInteger()) {
return true;
}
stack.pop();
for (int i = cur.getList().size() - 1; i >= 0; i--) {
stack.push(cur.getList().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();
*/