Minimum Cost of Ropes

The problem. You have n ropes with lengths ropes[i]. Joining two ropes of lengths a and b makes one rope of length a + b and costs a + b. Join all the ropes into one, and return the minimum total cost.

Example

ropes = [4, 3, 2, 6]   ->  29
ropes = [1, 2, 3]      ->  9       (1 + 2 = 3, then 3 + 3 = 6)
ropes = [2, 2, 3, 3]   ->  20      (2 + 2 = 4, 3 + 3 = 6, 4 + 6 = 10)
ropes = [5]            ->  0       (nothing to join)