1. Climbing Stairs
Can use an array to store all numbers or just use a few variables to store the previous two steps since the current step number is based on the previous two step numbers.
3. House Robber II
dp + 去头去尾
由于新增的限制条件是首尾的两个房子不能被同时闯入，那么可以分别把首尾的房子去掉，长度变成n - 1，各算一遍最大的值，然后取较大值就可以了。这样的话，就可以重复利用house robber的答案了
4. House Robber III
https://leetcode.com/problems/house-robber-iii/discuss/79330/Step-by-step-tackling-of-the-problem
这个解释的非常好，step by step