Add solution #331

Author: JoshCrozier

README.md

@@ -243,6 +243,7 @@
 322|[Coin Change](./0322-coin-change.js)|Medium|
 326|[Power of Three](./0326-power-of-three.js)|Easy|
 328|[Odd Even Linked List](./0328-odd-even-linked-list.js)|Medium|
+331|[Verify Preorder Serialization of a Binary Tree](./0331-verify-preorder-serialization-of-a-binary-tree.js)|Medium|
 334|[Increasing Triplet Subsequence](./0334-increasing-triplet-subsequence.js)|Medium|
 338|[Counting Bits](./0338-counting-bits.js)|Easy|
 342|[Power of Four](./0342-power-of-four.js)|Easy|

solutions/0331-verify-preorder-serialization-of-a-binary-tree.js

@@ -0,0 +1,38 @@
+/**
+ * 331. Verify Preorder Serialization of a Binary Tree
+ * https://leetcode.com/problems/verify-preorder-serialization-of-a-binary-tree/
+ * Difficulty: Medium
+ *
+ * One way to serialize a binary tree is to use preorder traversal. When we encounter a non-null
+ * node, we record the node's value. If it is a null node, we record using a sentinel value such
+ * as '#'.
+ *
+ * For example, the above binary tree can be serialized to the string "9,3,4,#,#,1,#,#,2,#,6,#,#",
+ * where '#' represents a null node.
+ *
+ * Given a string of comma-separated values preorder, return true if it is a correct preorder
+ * traversal serialization of a binary tree.
+ *
+ * It is guaranteed that each comma-separated value in the string must be either an integer or
+ * a character '#' representing null pointer.
+ *
+ * You may assume that the input format is always valid.
+ * For example, it could never contain two consecutive commas, such as "1,,3".
+ * Note: You are not allowed to reconstruct the tree.
+ */
+
+/**
+ * @param {string} preorder
+ * @return {boolean}
+ */
+var isValidSerialization = function(preorder) {
+  let result = 1;
+  for (const node of preorder.split(',')) {
+    if (result) {
+      result += node === '#' ? -1 : 1;
+    } else {
+      return false;
+    }
+  }
+  return result < 1;
+};