341. Flatten Nested List Iterator

You are given a nested list of integers nestedList. Each element is either an integer or a list whose elements may also be integers or other lists. Implement an iterator to flatten it.

Implement the NestedIterator class:

• NestedIterator(List nestedList) Initializes the iterator with the nested list nestedList.
• int next() Returns the next integer in the nested list.
• boolean hasNext() Returns true if there are still some integers in the nested list and false otherwise.

/**
 * // 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 {
    vector<int> vec;
    int idx = 0;

    void recursiveInit(const vector<NestedInteger> &nestedList) {
        for(auto& nest : nestedList) {
            if(nest.isInteger()) vec.push_back(nest.getInteger());
            else recursiveInit(nest.getList());
        }
    }
public:
    NestedIterator(vector<NestedInteger> &nestedList) {
        for(auto& nest : nestedList) {
            if(nest.isInteger()) vec.push_back(nest.getInteger());
            else recursiveInit(nest.getList());
        }
    }

    int next() {
        return vec[idx++];
    }

    bool hasNext() {
        return idx < vec.size();
    }
};

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