[leetcode]341. Flatten Nested List Iterator

题目链接：https://leetcode.com/problems/flatten-nested-list-iterator/?tab=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.

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)
    {
        vector<int> res;
        for(int i=0;i<nestedList.size();i++)
            flatten(nestedList[i]);
        k=0;
    }

    int next()
    {
        return flat[k++];
    }

    bool hasNext()
    {
        return k<flat.size();
    }
private:
    vector<NestedInteger> vec;
    vector<int> flat;
    int k=0;
    void flatten(NestedInteger n)
    {
        if(n.isInteger())
        {
            flat.push_back(n.getInteger());
            return;
        }
        const vector<NestedInteger> v=n.getList();
        for(int i=0;i<v.size();i++)
            flatten(v[i]);

    }
};

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