Read in a sequence of characters from standard input and maintain the characters in a linked list with no duplicates. When you read in a previously unseen character, insert it at the front of the list. When you read in a duplicate character, delete it from the list and reinsert it at the beginning. This implements the well-known move-to-front strategy, which is useful for caching, data compression, and many other applications where items that have been recently accessed are more likely to be reaccessed.