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].

【思路】建一个栈，从list尾巴倒序压入栈中，每次检测栈顶，如果是整数元素，直接弹出；如果是向量，则将整个向量先弹出，再倒序压入每一个向量中的元素。

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

    int next() {
        int val;
        if(hasNext())
        {
            val = stk.top().getInteger();
            stk.pop();
        }
        return val;
    }

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

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