337. House Robber III

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
/**
 * Recursive Greedy Solution.
 *
 * Time Complexity: O(N)
 *
 * Space Complexity: O(N)
 *
 * N = Number of nodes in the tree.
 */
class Solution {
    public int rob(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int[] result = robHelper(root);
        return Math.max(result[0], result[1]);
    }

    private int[] robHelper(TreeNode node) {
        if (node == null) {
            return new int[2];
        }

        int[] left = robHelper(node.left);
        int[] right = robHelper(node.right);

        int[] res = new int[2];
        res[0] = Math.max(left[0], left[1]) + Math.max(right[0], right[1]);
        res[1] = node.val + left[0] + right[0];

        return res;
    }
}