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.

• 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].
• 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].

 1 /**
 2  * // This is the interface that allows for creating nested lists.
 3  * // You should not implement it, or speculate about its implementation
 4  * public interface NestedInteger {
 5  *
 6  *     // @return true if this NestedInteger holds a single integer,
 7  *     // rather than a nested list.
 8  *     public boolean isInteger();
 9  *
10  *     // @return the single integer that this NestedInteger holds,
11  *     // if it holds a single integer
12  *     // Return null if this NestedInteger holds a nested list
13  *     public Integer getInteger();
14  *
15  *     // @return the nested list that this NestedInteger holds,
16  *     // if it holds a nested list
17  *     // Return null if this NestedInteger holds a single integer
18  *     public List<NestedInteger> getList();
19  * }
20  */
21 import java.util.Iterator;
22 import java.util.Stack;
23
24 public class NestedIterator implements Iterator<Integer> {
25
26     Stack<NestedInteger> stack = new Stack<NestedInteger>();
27
28     public NestedIterator(List<NestedInteger> nestedList) {
29         if (nestedList == null)
30             return;
31
32         for (int i = nestedList.size() - 1; i >= 0; i--) {
33             stack.push(nestedList.get(i));
34         }
35     }
36
37     // @return {int} the next element in the iteration
38     @Override
39     public Integer next() {
40         return stack.pop().getInteger();
41     }
42
43     // @return {boolean} true if the iteration has more element or false
44     @Override
45     public boolean hasNext() {
46         while (!stack.isEmpty()) {
47             NestedInteger top = stack.peek();
48             if (top.isInteger()) {
49                 return true;
50             } else {
51                 stack.pop();
52                 for (int i = top.getList().size() - 1; i >= 0; i--) {
53                     stack.push(top.getList().get(i));
54                 }
55             }
56         }
58         return false;
59     }
60
61     @Override
62     public void remove() {
63
64     }
65 }
66
67 /**
68  * Your NestedIterator object will be instantiated and called as such:
69  * NestedIterator i = new NestedIterator(nestedList); while (i.hasNext())
70  * v.add(i.next());
71  */

Flatten Nested List Iterator