1)编写一个程序，接受用户输入的一个字符串，统计其中出现过的所有字符，按其出现频率从高到低的顺序排列输出。(注：请使用这样的方法编写程序，使用一个链表存放出现过的字符，链表的每个结点的数据域中有两个变量，一个存放该字符，一个存放字符出现的次数。每检索到原字符串的一个字符时，先在链表中查找，若找到此字符，则将其出现次数加l，否则创建一个新的结点链接在链表的末尾。)

2)对已经建立好的链表进行如下处理：删除出现次数小于3的字符节点，并输出删除之后的链表信息。

-1) Write a program that accepts user input a string, statistics which appeared in all the characters, their frequency of occurrence in descending order of output. (Note: the use of such method of preparation, the use of a stored list of characters appeared, the linked list data field of each node has two variables, a storage of the character, a number of characters stored in each retrieved original string of one character, the first in the list to find, if it finds this character, it is the number of occurrences plus l, or create a new node in the list at the end of the link.) 2) have already been carried out to establish a good list handled as follows: Delete occurrences of characters is less than 3 nodes, and outputs the list after deletion of information.