337. House Robber III

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    Map<TreeNode, Integer> memo = new HashMap<>();
    
    public int rob(TreeNode root) {
        if (root == null) {
            return 0;
        }
        if (memo.get(root) != null) {
            return memo.get(root);
        }
        int robNow = root.val 
            + (root.left == null ? 0 : rob(root.left.left) + rob(root.left.right))
            + (root.right == null ? 0 : rob(root.right.left) + rob(root.right.right)); // rob next next
        
        int notRobNow = rob(root.left) + rob(root.right); // can rob next
        int result = Math.max(robNow, notRobNow);
        
        memo.put(root, result);
        return result;
    }
}
/*
rob or not rob

use memo
TC:O(n)
SC:O(n)

*/