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解法

/**
 * 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 {
    public int rob(TreeNode root) {
        int[] res = robTree(root);
        return Math.max(res[0], res[1]);
    }
   /**
    * 递归获取当前节点往下，取当前节点的最大值、不取当前节点的最大值
    */    
public int[] robTree(TreeNode root) {
        if (root == null) {
            return new int[]{0, 0};
        }
        int[] left = robTree(root.left);
        int[] right = robTree(root.right);
        // 取当前节点，不能再取子节点，能得到的最大值
        int val1 = root.val + left[0] + right[0];
        // 不取当前节点，子节点取不取都行，得到的最大值
        int val0 = Math.max(left[0], left[1]) + Math.max(right[0], right[1]); 
        return new int[]{val0, val1};
    }
}