leetcode-341 Flatten Nested List Iterator

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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.

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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].
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/**
 * // 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 {
private:
    stack<NestedInteger> s;
public:
    NestedIterator(vector<NestedInteger> &nestedList) {
        int len = nestedList.size();
        int i = 0;
        
        for(i = len-1; i>=0; i--){
            s.push(nestedList[i]);
        }
    }

    int next() {
        NestedInteger temp = s.top();
        s.pop();
        return temp.getInteger();
    }

    bool hasNext() {
        while(!s.empty()){
            NestedInteger temp = s.top();
            if(temp.isInteger())
                return true;
            else{
                s.pop();
                vector<NestedInteger> List = temp.getList();
                int len = List.size();
                int i = 0;
                
                for(i = len-1; i>=0; i--){
                    s.push(List[i]);
                }
            }
        }
        return false;
    }
};

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