Nested List Weight Sum

Given a nested list of integers, return the sum of all integers in the list weighted by their depth.

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]], return 10. (four 1's at depth 2, one 2 at depth 1)

Example 2:
Given the list [1,[4,[6]]], return 27. (one 1 at depth 1, one 4 at depth 2, and one 6 at depth 3; 1 + 4*2 + 6*3 = 27)

Analyse: Use recursion to solve this problem. 

Runtime: 3ms

/**
 * // 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:
    int depthSum(vector<NestedInteger>& nestedList) {
        int result = 0;
        helper(nestedList, 1, result);
        return result;
    }
    
    void helper(vector<NestedInteger>& nestedList, int depth, int& result) {
        for (int i = 0; i < nestedList.size(); i++) {
            if (nestedList[i].isInteger()) {
                result += depth * nestedList[i].getInteger();
            } else {
                vector<NestedInteger> newNestedList = nestedList[i].getList();
                helper(newNestedList, depth + 1, result);
            }
        }
    }
};