341. 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].

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *   public:
 *     // Return true if this NestedInteger holds a single integer, rather than a nested list.
 *     bool isInteger() const;
 *
 *     // Return the single integer that this NestedInteger holds, if it holds a single integer
 *     // The result is undefined if this NestedInteger holds a nested list
 *     int getInteger() const;
 *
 *     // Return the nested list that this NestedInteger holds, if it holds a nested list
 *     // The result is undefined if this NestedInteger holds a single integer
 *     const vector<NestedInteger> &getList() const;
 * };
 */

class NestedIterator {
public:
    NestedIterator(vector<NestedInteger> &nestedList) {
        helper(nestedList, data);
        if (data.size() != 0) {
            cur = 0;
        }
    }
    
    int next() {
        int ans;
        if (hasNext()) {
            ans = data[cur];
            cur++;
        }
        return ans;
    }
    
    bool hasNext() {
        return cur < data.size();
    }
    
private:
    void helper(vector<NestedInteger> &nestedList, vector<int>& data) {
        for (auto NI : nestedList) {
            if (NI.isInteger()) {
                data.push_back(NI.getInteger());
            } else {
                helper(NI.getList(), data);
            }
        }
    }
    vector<int> data;
    int cur;
};

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i(nestedList);
 * while (i.hasNext()) cout << i.next();
 */