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)

一刷：
题解，用recursion求解，子函数内带入自己的depth

/**
 * // 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 depth = 1;
        int res = 0;
        for(NestedInteger nested : nestedList){
            res += count(nested, depth);
        }
        return res;
    }
    
    private int count(NestedInteger nested, int depth){
        if(nested.isInteger()) return nested.getInteger()*depth;
        
        List<NestedInteger> lists = nested.getList();
        depth++;
        int res = 0;
        for(NestedInteger list : lists){
            res += count(list, depth);
        }
        return res;
    }
}

二刷
recursion

/**
 * // 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) {
        return helper(nestedList, 1);
    }
    
    private int helper(List<NestedInteger> nestedList, int depth){
        int res = 0;
        for(NestedInteger nest : nestedList){
            if(nest.isInteger()) res += nest.getInteger()*depth;
            else{
                List<NestedInteger> list = nest.getList();
                res += helper(list, depth+1);
            }
        }
        return res;
    }
}