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". Example 1: Input: preorder = "9,3,4,#,#,1,#,#,2,#,6,#,#" Output: true Example 2: Input: preorder = "1,#" Output: false Example 3: Input: preorder = "9,#,#,1" Output: false Constraints: 1 <= preorder.length <= 104 preoder consist of integers in the range [0, 100] and '#' separated by commas ','. Follow up: Find an algorithm without reconstructing the tree.