题目：https://leetcode.com/problems/word-frequency/
代码(github)：https://github.com/illuz/leetcode

统计 word.txt 里的单词和频数，然后按频数从小到大排序输出。

1. 用 awk 统计单词个数。
   • 用 awk 里的字典，循环统计每一行的第个单词。
   • 处理完全部行后，再用for (item in Dict) { #do someting# } 把单词和频数输出来。
2. 不过现在只是做好了统计，还要排序下。用管道 | 把输出交给 sort 来排序
   • sort -n 把字符串转成数来比较。
   • sort -r 从小到大。
   • sort -k 2 把第二个字串当做 key。

English version:

1. I should count the words. So I chose the awk command.

• I use a dictionary in awk. For every line I count every word in the dictionary.
• After deal with all lines. At the END, use for (item in Dict) { #do someting# } to print every words and its frequency.

2. Now the printed words are unsorted. Then I use a | pipes and sort it by sort

• sort -n means "compare according to string numerical value".
• sort -r means "reverse the result of comparisons".
• sort -k 2 means "sort by the second word"