LeetCode - Nested List Weight Sum

NestedListWeightSum.java

// 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)

/**
 * // 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 res = 0; 
        if(nestedList == null || nestedList.size() == 0) return res;
        res = helper(nestedList, 1);
        return res;
    }
    
    private int helper(List<NestedInteger> lst, int level){
        int res = 0;
        for(NestedInteger integer : lst){
            if(integer.isInteger()){
                res += integer.getInteger() * level;   
            } else {
                res += helper(integer.getList(), level + 1);
            }
        }
        return res;
    }
}