solution.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var maxSumBST = function (root) {
    const rst = postOrder(root)[0];
    return rst > 0 ? rst : 0;
};

function postOrder (root) {
    if (!root) {
        return [
            -Infinity, true, -Infinity, Infinity, 0, ];
    }
    if (!root.left && !root.right) {
        return [
            root.val, true, root.val, root.val, root.val, ];
    }
    let isBST = true;
    let result = -Infinity;
    let min = root.val;
    let max = root.val;
    let cur = root.val;
    if (root.left) {
        const [
            leftRst, leftIsBST, leftMin, leftMax, leftSum, ] = postOrder(root.left);
        result = Math.max(result, leftRst);
        if (!leftIsBST || root.val <= leftMax) {
            isBST = false;
        } else {
            cur += leftSum;
        }
        min = leftMin;
    }
    if (root.right) {
        const [
            rightRst, rightIsBST, rightMin, rightMax, rightSum, ] = postOrder(root.right);
        result = Math.max(result, rightRst);
        if (!rightIsBST || root.val >= rightMin) {
            isBST = false;
        } else {
            cur += rightSum;
        }
        max = rightMax;
    }
    return [
        Math.max(result, isBST ? cur : -Infinity), isBST, min, max, cur, ];
}