Echo Cancellation

A radio signal transmits characters one by one. Occasionally, an 'echo' occurs where the same character is repeated twice consecutively (e.g., 'aa'). When this happens, both characters cancel each other out and disappear. This can cause a chain reaction (e.g., in 'abbaf', 'bb' cancels, leaving 'aaf', then 'aa' cancels, leaving 'f'). Given a string, return the final signal after all possible cancellations. Example: Input: abccba Output: (empty string)