213. House Robber II

/**
 * Iterative Solution
 *
 * Refer:
 * https://leetcode.com/problems/house-robber-ii/discuss/59934/Simple-AC-solution-in-Java-in-O(n)-with-explanation/61023
 *
 * Time Complexity: O(N)
 *
 * Space Complexity: O(1)
 *
 * N = Length of the input nums array.
 */
class Solution {
    public int rob(int[] nums) {
        if (nums == null || nums.length == 0) {
            return 0;
        }
        int len = nums.length;
        if (len == 1) {
            return nums[0];
        }

        return Math.max(nums[0] + robHelper(nums, 2, len - 2), robHelper(nums, 1, len - 1));
    }

    private int robHelper(int[] nums, int left, int right) {
        int previousMax = 0;
        int currentMax = 0;
        for (int i = left; i <= right; i++) {
            int temp = currentMax;
            currentMax = Math.max(currentMax, nums[i] + previousMax);
            previousMax = temp;
        }
        return currentMax;
    }
}