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 + 42 + 63 = 27)

Solution

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * public interface NestedInteger {
 *
 *     // @return true if this NestedInteger holds a single integer, rather than a nested list.
 *     public boolean isInteger();
 *
 *     // @return the single integer that this NestedInteger holds, if it holds a single integer
 *     // Return null if this NestedInteger holds a nested list
 *     public Integer getInteger();
 *
 *     // @return the nested list that this NestedInteger holds, if it holds a nested list
 *     // Return null if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
public class Solution {
   public int depthSum(List<NestedInteger> nestedList) {
        int[] sum = new int[1];
        for(NestedInteger nestedInteger: nestedList) {
            helper(nestedInteger, sum, 1);
        }

        return sum[0];
    }

    private void helper(NestedInteger nestedInteger, int[] sum, int level) {
        if (nestedInteger.isInteger()) {
            sum[0] += nestedInteger.getInteger() * level;
        } else {
            List<NestedInteger> list = nestedInteger.getList();
            for(NestedInteger nestedInteger1 : list) {
                helper(nestedInteger1, sum, level+1);
            }
        }
    }
}