主要实现的功能是：任意输入一字符串，可以是任意的字符，包括标点符号，空格，以及键盘上能输入的所以字符。任意确定一个关键字符，统计该关键字符在字符串中出现的次数！ -Main function is to: a string of arbitrary input can be arbitrary characters, including punctuation, spaces, and the keyboard to input characters so. Arbitrarily defi

根据一个英文文本文件，不区分大小写的统计26个字母的出现频率。假设内存足够大，可以将文件一次性读入。(According to an English text file, the frequency of appearance of 26 letters is not case sensitive. Assuming that the memory is large enough, you can read the file one at a time.)