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

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].

Runtime: 626ms

 1 /**
 2  * // This is the interface that allows for creating nested lists.
 3  * // You should not implement it, or speculate about its implementation
 4  * class NestedInteger {
 5  *   public:
 6  *     // Return true if this NestedInteger holds a single integer,
 7  *     // rather than a nested list.
 8  *     bool isInteger() const;
 9  *
10  *     // Return the single integer that this NestedInteger holds,
11  *     // if it holds a single integer
12  *     // The result is undefined if this NestedInteger holds a nested list
13  *     int getInteger() const;
14  *
15  *     // Return the nested list that this NestedInteger holds,
16  *     // if it holds a nested list
17  *     // The result is undefined if this NestedInteger holds a single integer
18  *     const vector<NestedInteger> &getList() const;
19  * };
20  */
21 class NestedIterator {
22 public:
23     NestedIterator(vector<NestedInteger> &nestedList) {
24         // Initialize your data structure here.
25         flatten(nestedList, result);
26         index = 0;
27     }
28 
29     // @return {int} the next element in the iteration
30     int next() {
31         // Write your code here
32         return result[index++];
33     }
34 
35     // @return {boolean} true if the iteration has more element or false
36     bool hasNext() {
37         // Write your code here
38         return index < result.size();
39     }
40     
41 private:
42     vector<int> result;
43     int index;
44     
45     void flatten(vector<NestedInteger> nestedList, vector<int>& result) {
46         for (int i = 0; i < nestedList.size(); i++) {
47             if (nestedList[i].isInteger())
48                 result.push_back(nestedList[i].getInteger());
49             else 
50                 flatten(nestedList[i].getList(), result);
51         }
52     }
53 };
54 
55 /**
56  * Your NestedIterator object will be instantiated and called as such:
57  * NestedIterator i(nestedList);
58  * while (i.hasNext()) v.push_back(i.next());
59  */