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].

/**
 * // 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) {
        for (size_t i = nestedList.size(); i != 0; --i) {
            st.push(nestedList[i-1]);
        }
    }

    int next() {
        int ret = st.top().getInteger();
        st.pop();
        return ret;
    }

    bool hasNext() {
        while (!st.empty()) {
            if (st.top().isInteger()) return true;

            auto nestedList = st.top().getList();
            st.pop();
            for (size_t i = nestedList.size(); i != 0; --i) {
                st.push(nestedList[i-1]);
            }           
        }
        return false;
    }

private:
    stack<NestedInteger> st;
};