[day2]
1. Finish problem 3, a delete-in-the-middle problem. It is easy to delete the middle if we know the previous node. However, if we considered that we can delete the next node easily, and we can consider the current node is next node, and next node is the node we need to delete(since nodes are all the same). In this case, we just need to copy the data and then delete the next node.
2. Coner cases, 1. nodes in the middle, 2, nodes in the head, 3, node in the tail, 4, node nullptr; first 2 is valid, following two is invalid/