Triangle 

Given a triangle, find the minimum path sum from top to bottom. Each
step you may move to adjacent numbers on the row below.

For example, given the following triangle
[
     [2],
    [3,4],
   [6,5,7],
  [4,1,8,3]
]
The minimum path sum from top to bottom is 11 (i.e., 2 + 3 + 5 + 1 =
11).

Note:
Bonus point if you are able to do this using only O(n) extra space,
where n is the total number of rows in the triangle.