函数fun的功能是计算出链表中各结点数据域中值之和，由于链表单向，利用循环从头结点开始搜索直到最后一个结点，每找到一个结点就把结点的数据域上的数据累加起来。-Function fun feature is calculated in the node list of data field values and, as a one-way linked list, nodes from scratch using loop start searching until the last node,