One way to serialize a binary tree is to use pre-order 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 #.
     _9_
    /   \
   3     2
  / \   / \
 4   1  #  6
/ \ / \   / \
# # # #   # #
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, verify whether it is a correct preorder traversal serialization of a binary tree. Find an algorithm without reconstructing the tree.
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: "9,3,4,#,#,1,#,#,2,#,6,#,#"
Output: true
Example 2:
Input: "1,#"
Output: false
Example 3:
Input: "9,#,#,1"
Output: false
class Solution:
     def isValidSerialization(self, preorder):
         """
         :type preorder: str
         :rtype: bool
         """
         arr = preorder.split(',')
         k = len(arr)
         for _ in range(k):
             print(arr)
             if arr==['#']:
                 return True
             newArr = []
             i = 0
             while i < len(arr):
                 if i < len(arr)-2 and arr[i].isdigit() and arr[i+1:i+3] == ['#','#']:
                     newArr += ['#']
                     i += 3
                 else:
                     newArr += [arr[i]]
                     i += 1
             arr = newArr
         return False

class Solution:
     def isValidSerialization(self, preorder):
         """
         :type preorder: str
         :rtype: bool
         """
         arr = preorder.split(',')
         k = len(arr)
         for _ in range(k):
             print(arr)
             if arr==['#']:
                 return True
             newArr = []
             i = 0
             while i < len(arr):
                 if i < len(arr)-2 and arr[i].isdigit() and arr[i+1:i+3] == ['#','#']:
                     newArr += ['#']
                     i += 3
                 else:
                     newArr += [arr[i]]
                     i += 1
             arr = newArr
         return False
