Flatten Nested List Iterator

 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 
23 public class NestedIterator implements Iterator<Integer> {
24     
25     Stack<NestedInteger> stack;
26     public NestedIterator(List<NestedInteger> nestedList) {
27         // Initialize your data structure here.
28         stack = new Stack<NestedInteger>();
29         if(nestedList!=null){
30             for(int i=nestedList.size()-1;i>=0;i--){
31                 stack.push(nestedList.get(i));
32             }
33         }
34     }
35 
36     // @return {int} the next element in the iteration
37     @Override
38     public Integer next() {
39         // Write your code here
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         // Write your code here
47         while(!stack.empty()){
48             NestedInteger cur = stack.peek();
49             if(cur.isInteger()) return true;
50             stack.pop();
51             List<NestedInteger> list = cur.getList();
52             if(list!=null){
53                 for(int i=list.size()-1;i>=0;i--){
54                     stack.push(list.get(i));
55                 }
56             }
57         }
58         return false;
59     }
60 
61     @Override
62     public void remove() {}
63 }
64 
65 /**
66  * Your NestedIterator object will be instantiated and called as such:
67  * NestedIterator i = new NestedIterator(nestedList);
68  * while (i.hasNext()) v.add(i.next());
69  */