Design-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:
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].

class NestedIterator {
public:
    NestedIterator(vector<NestedInteger> &nestedList) {
        flat(nestedList);
        listIter = mList.begin();
    }  

    int next() {
        int res = *listIter;
        ++listIter;
        return res;
    }  

    bool hasNext() {
        return listIter != mList.end();
    }  

    void flat(vector<NestedInteger>& nL){
        for (auto &it : nL) {
            if (it.isInteger())
                mList.push_back(it.getInteger());
            else
                flat(it.getList());
        }
    }  

private:
    list<int>::iterator listIter;
    list<int>mList;
};