leetcode 341. 扁平化嵌套列表迭代器(Flatten Nested List Iterator)

题目描述：

给定一个嵌套的整型列表。设计一个迭代器，使其能够遍历这个整型列表中的所有整数。

列表中的项或者为一个整数，或者是另一个列表。

示例 1:

输入: [[1,1],2,[1,1]]
输出: [1,1,2,1,1]
解释: 通过重复调用 next 直到 hasNext 返回false，next 返回的元素的顺序应该是: [1,1,2,1,1]。

示例 2:

输入: [1,[4,[6]]]
输出: [1,4,6]
解释: 通过重复调用 next 直到 hasNext 返回false，next 返回的元素的顺序应该是: [1,4,6]。

---

解法：

/**
 * // 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:
    stack<int> stk;
    void unpack(stack<int>& stk, vector<NestedInteger>& lst){
        int len = lst.size();
        for(int j = len-1; j >= 0; j--){
            NestedInteger nst = lst[j];
            if(nst.isInteger()){
                stk.push(nst.getInteger());
            }else{
                vector<NestedInteger> lst = nst.getList();
                unpack(stk, lst);
            }
        }
        
    }
    NestedIterator(vector<NestedInteger> &nestedList) {
        unpack(stk, nestedList);
    }

    int next() {
        int res = stk.top();
        stk.pop();
        return res;
    }

    bool hasNext() {
        return !stk.empty();
    }
};

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