Create 1373-maximum-sum-bst-in-binary-tree.js

Author: everthis

1373-maximum-sum-bst-in-binary-tree.js

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+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val, left, right) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.left = (left===undefined ? null : left)
+ *     this.right = (right===undefined ? null : right)
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {number}
+ */
+const maxSumBST = function (root) {
+  let maxSum = 0
+  postOrderTraverse(root)
+  return maxSum
+
+  function postOrderTraverse(root) {
+    if (root == null) return [Number.MAX_VALUE, -Infinity, 0] // {min, max, sum}, initialize min=MAX_VALUE, max=MIN_VALUE
+    let left = postOrderTraverse(root.left)
+    let right = postOrderTraverse(root.right)
+    // The BST is the tree:
+    if (
+      !(
+        left != null && // the left subtree must be BST
+        right != null && // the right subtree must be BST
+        root.val > left[1] && // the root's key must greater than maximum keys of the left subtree
+        root.val < right[0]
+      )
+    )
+      // the root's key must lower than minimum keys of the right subtree
+      return null
+    let sum = root.val + left[2] + right[2] // now it's a BST make `root` as root
+    maxSum = Math.max(maxSum, sum)
+    let min = Math.min(root.val, left[0])
+    let max = Math.max(root.val, right[1])
+    return [min, max, sum]
+  }
+}