You are given a nested list of integers `nestedList`. Each element is either an integer or a list whose elements may also be integers or other lists. Implement an iterator to flatten it.

Implement the `NestedIterator` class:

- `NestedIterator(List<NestedInteger> nestedList)` Initializes the iterator with the nested list `nestedList`.
- `int next()` Returns the next integer in the nested list.
- `boolean hasNext()` Returns `true` if there are still some integers in the nested list and `false` otherwise.

Your code will be tested with the following pseudocode:

    initialize iterator with nestedList
    res = []
    while iterator.hasNext()
        append iterator.next() to the end of res
    return res

If `res` matches the expected flattened list, then your code will be judged as correct.

Example 1:

    Input: nestedList = [[1,1],2,[1,1]]
    Output: [1,1,2,1,1]
    Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].

Example 2:

    Input: nestedList = [1,[4,[6]]]
    Output: [1,4,6]
    Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].

Constraints:

- `1 <= nestedList.length <= 500`
- The values of the integers in the nested list is in the range `[-106, 106]`.