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## Problem

The thief has found himself a new place for his thievery again. There is only one entrance to this area, called the “root.” Besides the root, each house has one and only one parent house. After a tour, the smart thief realized that “all houses in this place forms a binary tree”. It will automatically contact the police if two directly-linked houses were broken into on the same night.

Determine the maximum amount of money the thief can rob tonight without alerting the police.

Example 1:

Input: [3,2,3,null,3,null,1]

3
/ \
2   3
\   \
3   1

Output: 7
Explanation: Maximum amount of money the thief can rob = 3 + 3 + 1 = 7.


Example 2:

Input: [3,4,5,1,3,null,1]

3
/ \
4   5
/ \   \
1   3   1

Output: 9
Explanation: Maximum amount of money the thief can rob = 4 + 5 = 9.


## Code

class Solution {
public int[] helper(TreeNode node) {
if (node == null) {
return new int[] { 0, 0 };
}

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

int rob = node.val + left + right;
int notRob = Math.max(left, left) + Math.max(right, right);

return new int[] { rob, notRob };
}

public int rob(TreeNode root) {
int[] res = helper(root);
return Math.max(res, res);
}
}