You are given a String s. In increasing order, consider each integer i between 1 and the length of s, inclusive. For each value of i, you can choose whether or not to perform the following operation: Take the prefix of length i of the current string and reverse it. Your ultimate goal is to end up with the alphabetically earliest string possible.
For example, if s = "BCDAF", you should reverse the prefix of length 3 to get "DCBAF", and then reverse the prefix of length 4 to get "ABCDF".
Return the alphabetically earliest string possible.