339. 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)

这道题要用草稿纸拿例子演示几次，才能发现level应该在遍历到整数的时候乘，而在遍历到list的时候不乘。并且在遍历到list的时候，调用递归函数要记得level + 1.

/**
 * // 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 getList();
 * }
 */
public class Solution {
    public int depthSum(List nestedList) {
        return helper(nestedList, 1);
    }
    private int helper(List nestedList, int level){
        int sum = 0;
        for (int i = 0; i < nestedList.size(); i++){
            if (nestedList.get(i).isInteger()){
                sum += level * nestedList.get(i).getInteger();
            } else {
                List list = nestedList.get(i).getList();
                sum += helper(list, level + 1);
            }
        }
        return sum;
    }
}