22.h

/**
 * // 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 Solution {
public:
    // @param nestedList a list of NestedInteger
    // @return a list of integer
    vector<int> flatten(vector<NestedInteger> &nestedList) {
        // Write your code here
        vector<int> r;
        flatten(nestedList, r);
        return r;
    }
    
    void flatten(const vector<NestedInteger> &list, vector<int> &r){
        for(int i = 0; i < list.size(); i++){
            if(!list[i].isInteger()){
                flatten(list[i].getList(), r);
            }
            else{
                r.push_back(list[i].getInteger());
            }
        }
    }
};