Create 1932-merge-bsts-to-create-single-bst.js

Author: everthis

1932-merge-bsts-to-create-single-bst.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[]} trees
+ * @return {TreeNode}
+ */
+const canMerge = function (trees) {
+  const mapRoots = {}
+  const mapLeaves = {}
+  let prev
+
+  //check all trees and hashMap all available roots and leaves
+  for (let node of trees) {
+    mapRoots[node.val] = node
+    if (node.left != null) {
+      if (mapLeaves[node.left.val] != null)
+        //different nodes can't refer to the same node -> abnormal BST
+        return null
+      mapLeaves[node.left.val] = node.left
+    }
+    if (node.right != null) {
+      if (mapLeaves[node.right.val] != null)
+        //different nodes can't refer to the same node -> abnormal BST
+        return null
+      mapLeaves[node.right.val] = node.right
+    }
+  }
+
+  let rootRes = null
+  let count = trees.length
+
+  //find potential root-result of the merged entire tree
+  //that is node without any references from the parent leaf nodes
+  for (let node of trees) {
+    if (mapLeaves[node.val] == null) {
+      rootRes = node
+      break
+    }
+  }
+
+  //if there are no nodes like that -> abnormal BST
+  if (rootRes == null) return rootRes
+
+  const q = []
+
+  //put root-result leaves into queue
+  if (rootRes.left != null) q.push(rootRes.left)
+  if (rootRes.right != null) q.push(rootRes.right)
+  count--
+
+  while (q.length) {
+    //get leaf from the queue and check if there is correponding available root
+    let leaf = q.pop()
+    let root = mapRoots[leaf.val]
+    if (root != null) {
+      //there is root matched to leaf, so let's merge it
+      count--
+      leaf.left = root.left
+      leaf.right = root.right
+      //add new leaves into the queue
+      if (root.left != null) q.push(root.left)
+      if (root.right != null) q.push(root.right)
+    }
+  }
+
+  prev = 0
+  //if we have merged all inputed trees and that is valid BST by values, then return rootRes
+  return count == 0 && recSanity(rootRes) ? rootRes : null
+
+  function recSanity(node) {
+    if (node == null) return true
+
+    if (!recSanity(node.left)) return false
+
+    if (prev >= node.val) return false
+    prev = node.val
+
+    return recSanity(node.right)
+  }
+}