Reverse Shuffle Merge

Given a string, A, we define some operations on the string as follows: a. denotes the string obtained by reversing string . Example: b. denotes any string that's a permutation of string . Example: c. denotes any string that's obtained by interspersing the two strings & , maintaining the order of characters in both. For example, & , one possible result of could be , another could be , another could be and so on. Given a string such that for some string , find the lexico