lk1373.java

/**
 * 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 {
    int maxSum = 0;

    public int maxSumBST(TreeNode root) {
        traverse(root);
        return maxSum;
    }

    // 函数返回 int[]{ isBST, min, max, sum}
    int[] traverse(TreeNode root) {
        // base case
        if (root == null) {
            return new int[]{
                    1, Integer.MAX_VALUE, Integer.MIN_VALUE, 0
            };
        }
        int[] left = traverse(root.left);
        int[] right = traverse(root.right);

        int[] res = new int[4];
        if (left[0] == 1 && right[0] == 1 &&
                root.val > left[2] && root.val < right[1]) {
            res[0] = 1;
            res[1] = Math.min(left[1], root.val);
            res[2] = Math.max(right[2], root.val);
            res[3] = left[3] + right[3] + root.val;
            maxSum = Math.max(maxSum, res[3]);
        } else {
            res[0] = 0;
        }
        return res;
    }
}