Problem description:

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:
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].

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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].

Solution:

first, we can store all the elements in reverse order with a stack. The reason that we need to use reverse order is because Stack is Last in First out. We can use the following example to explain.

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index:  0 1 2 3 4 5
data:   a b e d c q

        ___________
stack   a b e d c q|
        -----------
        |         |  
      last push   |
              first push

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ex: queue [10, 20], list [0, -10, -20]
queue.extendleft(list) -> [-20, -10, 0, 10, 20]
but we want list to be original order
queue.extendlist(list[::-1]) -> [0, -10, -20, 10, 20]
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class NestedIterator:

    def __init__(self, nestedList):
        self.queue = collections.deque(nestedList)
        
    def next(self):
        return self.queue.popleft().getInteger()

    def hasNext(self):
        while self.queue:
            if self.queue[0].isInteger():
                return True
            first = self.queue.popleft()
            self.queue.extendleft(first.getList()[::-1])
        return False
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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 {
public:
    NestedIterator(vector<NestedInteger> &nestedList) {
        //because the stack is LIFO, so if we want to do A item in stack first, then we need to push item A in the last
        for (int i = nestedList.size() - 1; i >= 0; --i) { 
            s.push(nestedList[i]);
        }
    }

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

    bool hasNext() {
        while (!s.empty()) {
            NestedInteger t = s.top(); 
            if (t.isInteger()) return true; //if this element is a number, return true and let next() do the job
            s.pop(); //this element is a list
            for (int i = t.getList().size() - 1; i >= 0; --i) {
                s.push(t.getList()[i]);
            }
        }
        return false;
    }

private:
    stack<NestedInteger> s;
};

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

reference:
https://goo.gl/Ut4Pns