Exercise 5: Eliminate Duplicates

Problem statement

This chapter explains how to use dictionaries for fast lookup of items (words, in our case). Keep a dictionary of the words that have been seen before, with their numbers as values. When adding a new word to the dictionary, assign the next number to it. Use the dictionary and formatted string to replace the known words with their numbers.

Task

Given text as a string, replace all duplicate occurrences of a word after the first with a unique number that represents that word, prefixed with the hash sign (#).

Example

The input "I am Sam I am Sam Sam I am" will become "I am Sam #0 #1 #2 #2 #0 #1".

Note: Assume that the text consists only of letters and spaces, with no punctuation.