Create Construct_Binary_Tree_from_Preorder_and_Inorder_Traversal.js

Author: ignacio-chiazzo

LeetcodeProblems/Construct_Binary_Tree_from_Preorder_and_Inorder_Traversal.js

@@ -0,0 +1,71 @@
+/*
+Construct Binary Tree from Preorder and Inorder Traversal
+https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/
+
+Given preorder and inorder traversal of a tree, construct the binary tree.
+
+Note:
+You may assume that duplicates do not exist in the tree.
+
+For example, given
+
+preorder = [3,9,20,15,7]
+inorder = [9,3,15,20,7]
+Return the following binary tree:
+
+    3
+   / \
+  9  20
+    /  \
+   15   7
+*/
+
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {number[]} preorder
+ * @param {number[]} inorder
+ * @return {TreeNode}
+ */
+var buildTree = function(preorder, inorder) {
+  if(preorder === null || inorder === null || preorder.length !== inorder.length)
+      return nil;
+  
+  return buildTreeAux(preorder, 0, preorder.length - 1, inorder, 0, inorder.length - 1);
+};
+
+var buildTreeAux = function(preorder, pl, ph, inorder, il, ih) {
+  if(pl > ph || il > ih)
+      return null;
+  
+  const rootValue = preorder[pl];
+  var countElementsLeft = 0;
+  while(inorder[il + countElementsLeft] !== rootValue) {
+    countElementsLeft++;
+  }
+    
+  var ret = new TreeNode(rootValue);
+  ret.left = buildTreeAux(preorder, pl + 1, pl + countElementsLeft, inorder, il, il + countElementsLeft - 1);
+  ret.right = buildTreeAux(preorder, pl + countElementsLeft + 1, ph, inorder, il + countElementsLeft + 1, ih);
+  
+  return ret;
+}
+
+var TreeNode = function(val) {
+  this.val = val;
+  this.left = this.right = null;
+}
+
+console.log(
+  buildTree(
+    [3,9,20,15,7],
+    [9,3,15,20,7]
+  )
+);
+
+module.exports.main = main