from the second layer, update the layer number every time.

from example.the following.

```
[
[2],
[3,4],
[6,5,7],
[4,1,8,3]
]
```

the first layer is 2. update the second the second layer [3+2, 4+2] = [ 5, 6]. then update the third layer

[6+5, 5+min(5, 6), 7+6] = [11, 10, 13], then the forth layer is [4+11,1+min(11, 10), 8 + min(5, 7), 3+7] = [15, 11, 13, 10], then the result is 11.

the code is like this:

```
class Solution(object):
def minimumTotal(self, triangle):
"""
:type triangle: List[List[int]]
:rtype: int
"""
if not triangle:
return 0
for i in xrange(1, len(triangle)):
triangle[i][0] += triangle[i-1][0]
triangle[i][-1] += triangle[i-1][-1]
for j in xrange(1, i):
triangle[i][j] += min(triangle[i-1][j-1], triangle[i-1][j])
return min(triangle[-1])
```