Add solution #889

Author: JoshCrozier

Diff for: README.md

@@ -420,6 +420,7 @@
 875|[Koko Eating Bananas](./0875-koko-eating-bananas.js)|Medium|
 876|[Middle of the Linked List](./0876-middle-of-the-linked-list.js)|Easy|
 884|[Uncommon Words from Two Sentences](./0884-uncommon-words-from-two-sentences.js)|Easy|
+889|[Construct Binary Tree from Preorder and Postorder Traversal](./0889-construct-binary-tree-from-preorder-and-postorder-traversal.js)|Medium|
 890|[Find and Replace Pattern](./0890-find-and-replace-pattern.js)|Medium|
 901|[Online Stock Span](./0901-online-stock-span.js)|Medium|
 905|[Sort Array By Parity](./0905-sort-array-by-parity.js)|Easy|

Diff for: solutions/0889-construct-binary-tree-from-preorder-and-postorder-traversal.js

@@ -0,0 +1,43 @@
+/**
+ * 889. Construct Binary Tree from Preorder and Postorder Traversal
+ * https://leetcode.com/problems/construct-binary-tree-from-preorder-and-postorder-traversal/
+ * Difficulty: Medium
+ *
+ * Given two integer arrays, preorder and postorder where preorder is the preorder traversal of a
+ * binary tree of distinct values and postorder is the postorder traversal of the same tree,
+ * reconstruct and return the binary tree.
+ *
+ * If there exist multiple answers, you can return any of them.
+ */
+
+/**
+ * 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 {number[]} preorder
+ * @param {number[]} postorder
+ * @return {TreeNode}
+ */
+var constructFromPrePost = function(preorder, postorder) {
+  if (!preorder.length) return null;
+
+  const root = new TreeNode(preorder[0]);
+  if (preorder.length === 1) return root;
+
+  const leftSize = postorder.indexOf(preorder[1]) + 1;
+  root.left = constructFromPrePost(
+    preorder.slice(1, leftSize + 1),
+    postorder.slice(0, leftSize)
+  );
+  root.right = constructFromPrePost(
+    preorder.slice(leftSize + 1),
+    postorder.slice(leftSize, -1)
+  );
+
+  return root;
+};