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].

 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, rather than a nested list.
 7  *     public boolean isInteger();
 8  *
 9  *     // @return the single integer that this NestedInteger holds, if it holds a single integer
10  *     // Return null if this NestedInteger holds a nested list
11  *     public Integer getInteger();
12  *
13  *     // @return the nested list that this NestedInteger holds, if it holds a nested list
14  *     // Return null if this NestedInteger holds a single integer
15  *     public List<NestedInteger> getList();
16  * }
17  */
18 public class NestedIterator implements Iterator<Integer> {
19     Stack<NestedInteger> st;
20     
21     public NestedIterator(List<NestedInteger> nestedList) {
22         st = new Stack<NestedInteger>();
23         for (int i=nestedList.size()-1; i>=0; i--) {
24             st.push(nestedList.get(i));
25         }
26     }
27 
28     @Override
29     public Integer next() {
30         return st.pop().getInteger();
31     }
32 
33     @Override
34     public boolean hasNext() {
35         while (!st.isEmpty()) {
36             if (st.peek().isInteger()) return true;
37             List<NestedInteger> cur = st.pop().getList();
38             for (int i=cur.size()-1; i>=0; i--) {
39                 st.push(cur.get(i));
40             }
41         }
42         return false;
43     }
44 }
45 
46 /**
47  * Your NestedIterator object will be instantiated and called as such:
48  * NestedIterator i = new NestedIterator(nestedList);
49  * while (i.hasNext()) v[f()] = i.next();
50  */