LeetCode [331] Verify Preorder Serialization of a Binary Tree

Ref
[1] https://leetcode.com/problems/verify-preorder-serialization-of-a-binary-tree/
[2] https://leetcode.com/discuss/83824/7-lines-easy-java-solution
[3] https://leetcode.com/discuss/83825/simple-python-solution-using-stack-with-explanation