Consider a large, unsorted file of records that contains many duplicates of the same records. One record is a duplicate of another if it has the same record number, where the record number is always six digits long. Consider the problem of removing all of the duplicates, so that the file contains a unique copy of each record. Devise an algorithm that removes all duplicates and runs in O(n) time, where n is the number of records in the file. Don't give pseudocode, just briefly describe the main idea of how the algorithm works.