House Robber

If we rob the curr house, we cannot rob curr-1 house.

Keep max_2_house_before as the amount we can get if we do NOT rob curr-1, i.e. we can rob curr.
Keep adjacent as the amount we can get if we rob curr-1, i.e. we cannot rob curr.
Althouge we can rob curr, we do NOT necessarily rob it. We can leave 2 house not robbed to get better result. In this case, we keep adjacent instead of max_2_house_before + curr.
Complexity: O(n)
```
def rob(self, nums):
 """
 :type nums: List[int]
 :rtype: int
 """
 max_2_house_before, adjacent = 0, 0
 for cur in nums:
     max_2_house_before, adjacent = \
     adjacent, max(adjacent, max_2_house_before+cur)
 return max(max_2_house_before, adjacent)
```
When the houses are in a circle. We can only rob one of the first and the last house. It is the same as rob nums[1:] and nums[:-1].