331. Verify Preorder Serialization of a Binary Tree

Runtime: 4 ms, faster than 79.18% of others
Memory Usage: 7 MB, less than 51.45% of others

和preorder traversal相似的概念，只是不用stack改用count去計算，如果是非空節點則+1，否則-1。不用C++內建的find去找下個逗號後，Runtime就快了3ms，不過Memory Usage卻多了0.2MB，完全不知道為什麼XD