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
[ , [3,4], [6,5,7], [4,1,8,3] ]
The minimum path sum from top to bottom is
2 + 3 + 5 + 1 = 11).
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.