Implement lossless dictionary encoding and decoding

Dictionary encoding is a lossless compression technique for a list of tokens. You are given an input string consisting of words separated by commas, e.g. - Input: `"USA,USA,USA,USA,Mexico,Canada,Mexico,Mexico,Mexico"` Implement two functions: 1. `encode(input_string) -> compressed_string` - Build a dictionary of unique words in the order of first appearance. - Replace each word occurrence with its dictionary index (0-based). - Output a single string in the format: - `dictionary_part:index_part` - `dictionary_part` is the unique words joined by commas. - `index_part` is the indices (as integers) joined by commas. - Important: The decoder will receive **only** the `compressed_string`; you may not pass any additional data. 2. `decode(compressed_string) -> original_string` - Reconstruct the exact original comma-separated string. Example: - Input: `"USA,USA,USA,USA,Mexico,Canada,Mexico,Mexico,Mexico"` - Encoded: `"USA,Mexico,Canada:0,0,0,0,1,2,1,1,1"` - Decoded: `"USA,USA,USA,USA,Mexico,Canada,Mexico,Mexico,Mexico"` Handle empty input strings appropriately.