Add Q3
Add Q2 answer
Replace tabs with spaces in Q2 answer
Remove global var, add test cases.
Replace struct name node with Node, fix bug that checks wrong head address.
- change to pass in head pointer in contains() and print_nodes().
- change pop() to return int value to avoid memory leak.
- change contains() to return int value to comply with const Node *head.
- rename tmp to meaningful names.
- change to use if(ptr) or (!ptr) for null pointer check.
- add print_case() for clear readability.