Add solution #971

Author: JoshCrozier

README.md

@@ -1,4 +1,4 @@
-# 1,054 LeetCode solutions in JavaScript
+# 1,055 LeetCode solutions in JavaScript
 
 [https://leetcode.com/](https://leetcode.com/)
 
@@ -779,6 +779,7 @@
 968|[Binary Tree Cameras](./solutions/0968-binary-tree-cameras.js)|Hard|
 969|[Pancake Sorting](./solutions/0969-pancake-sorting.js)|Medium|
 970|[Powerful Integers](./solutions/0970-powerful-integers.js)|Easy|
+971|[Flip Binary Tree To Match Preorder Traversal](./solutions/0971-flip-binary-tree-to-match-preorder-traversal.js)|Medium|
 976|[Largest Perimeter Triangle](./solutions/0976-largest-perimeter-triangle.js)|Easy|
 977|[Squares of a Sorted Array](./solutions/0977-squares-of-a-sorted-array.js)|Easy|
 985|[Sum of Even Numbers After Queries](./solutions/0985-sum-of-even-numbers-after-queries.js)|Easy|

solutions/0971-flip-binary-tree-to-match-preorder-traversal.js

@@ -0,0 +1,49 @@
+/**
+ * 971. Flip Binary Tree To Match Preorder Traversal
+ * https://leetcode.com/problems/flip-binary-tree-to-match-preorder-traversal/
+ * Difficulty: Medium
+ *
+ * You are given the root of a binary tree with n nodes, where each node is uniquely assigned a
+ * value from 1 to n. You are also given a sequence of n values voyage, which is the desired
+ * pre-order traversal of the binary tree.
+ *
+ * Any node in the binary tree can be flipped by swapping its left and right subtrees. For
+ * example, flipping node 1 will have the following effect.
+ *
+ * Flip the smallest number of nodes so that the pre-order traversal of the tree matches voyage.
+ *
+ * Return a list of the values of all flipped nodes. You may return the answer in any order.
+ * If it is impossible to flip the nodes in the tree to make the pre-order traversal match
+ * voyage, return the list [-1].
+ */
+
+/**
+ * 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
+ * @param {number[]} voyage
+ * @return {number[]}
+ */
+var flipMatchVoyage = function(root, voyage) {
+  const flips = [];
+  let index = 0;
+  return traverse(root) ? flips : [-1];
+
+  function traverse(node) {
+    if (!node) return true;
+    if (node.val !== voyage[index++]) return false;
+
+    if (!node.left) return traverse(node.right);
+    if (node.left.val === voyage[index]) return traverse(node.left) && traverse(node.right);
+
+    flips.push(node.val);
+    [node.left, node.right] = [node.right, node.left];
+    return traverse(node.left) && traverse(node.right);
+  }
+};