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.

Input: 

[[1,1],2,[1,1]]

Output: 

10 

Explanation: 

Four 1's at depth 2, one 2 at depth 1.

Input: 

[1,[4,[6]]]

Output: 

27 

Explanation: 

One 1 at depth 1, one 4 at depth 2, and one 6 at depth 3; 1 + 4*2 + 6*3 = 27.