Flatten Nested List Iterator.md

#Flatten Nested List Iterator ##Problem: 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]. ##Idea: Stack

/**
 * // 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(int i=(int)nestedList.size()-1;i>=0;i--)
        {
            nestedStack.push(nestedList[i]);
        }
    }

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

    bool hasNext() {
        while(!nestedStack.empty())
        {
            NestedInteger top = nestedStack.top();
            if(top.isInteger()) return true;
            else
            {
                nestedStack.pop();
                vector<NestedInteger>& topList=top.getList();
                for(int i=(int)topList.size()-1;i>=0;i--)
                {
                    nestedStack.push(topList[i]);
                }
            }
        }
        return false;
    }
private:
    stack<NestedInteger> nestedStack;
};

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