213. House Robber II

class Solution {
public:
    int rob(vector<int>& nums) {
        int n = nums.size();

        if (n == 0) return 0;
        if (n == 1) return nums[0];

        int max1 = helper(nums, 0, n - 1);
        int max2 = helper(nums, 1, n);

        return max(max1, max2);
    }
    int helper(vector<int>& nums, int left, int right) {
        int rob = 0, notRob = 0;
        for (int i = left; i < right; ++i) {
            int temp = rob;
            rob = max(nums[i] + notRob, rob);
            notRob = temp;
        }
        return rob;
    }
};

• T: $O(N)$
• S: $O(1)$