ZOJ Problem Set - 1888
Harvard linguistics professor George Kingsley Zipf (1902-1950) observed that the frequency of the kth most common word in a text is roughly proportional to 1/k. He justified his observations in a book titled Human behavior and the principle of least effort published in 1949. While Zipf's rationale has largely been discredited, the principle still holds, and others have afforded it a more sound mathematical basis.
You are to find all the words occurring n times in an English text. A word is a sequence of letters. Words are separated by non-letters. Capitalization should be ignored. A word can be of any length that an English word can be.
Leave a blank line between cases.
In practice, the difference between theory and practice is always
Man will occasionally stumble over the truth, but most of the
Source: University of Waterloo Local Contest 2001.06.02