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

/**
 * 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;
};