向链表中新增一个节点的小程序。分成三种情况：新节点前面有节点后面也有节点，新节点前面没有节点后面有节点，新节点前面有节点后面没有节点-A small program to add a node to the list. Divided into three cases: the new node in front of the node is also a node, the new node in front of the node has no node, the new node has no node in front of the node