Flatten Nested List Iterator

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:

Input: [[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: [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]

In the constructor, we push all the nested list into the stack from back to front, so when we pop the stack, it returns the very first element. 

/**
 * // 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;

  public NestedIterator(List<NestedInteger> nestedList) {
    stack = new Stack<>();
    populate(nestedList);
  }

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

  @Override
  public boolean hasNext() {
    return stack.size() > 0;
  }

  private void populate(List < NestedInteger > nestedList) {
    if (nestedList != null && !nestedList.isEmpty()) {
      for (int i = nestedList.size() - 1; i >= 0; i--) {
        NestedInteger nestedInteger = nestedList.get(i);
        if (nestedInteger.isInteger()) {
          stack.push(nestedInteger);
        } else {
          populate(nestedInteger.getList());
        }
      }
    }
  }
}
/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i = new NestedIterator(nestedList);
 * while (i.hasNext()) v[f()] = i.next();
 */