[LeetCode]Uncaught TypeError变量序列化错误解决

解决办法：删除变量中的特殊符号，包括合法的下划线

报错信息： 

Line 9: PHP Fatal error: Uncaught TypeError: Argument 1 passed to Serializer::serializeArray() must be of the type array, null given, called in /leetcode/precompiled/serializer.php on line 100 and defined in serializer.php