Reverse Only Letters in a String

You are given a string S consisting of letters and digits. Your task is to reverse the order of the letters in S while keeping the digits in their original positions.

For example, given the input string ab12cd34ef, the letters when reversed become fe, dc, ba while the digits 1,2,3,4 remain in the same positions, so the output is fe12dc34ba.

The reversal logic only applies to alphabetic characters (A–Z and a–z). Digits and any other characters, if present, should not be moved.

Note: If there are no letters, the string remains unchanged.

You can mathematically define the process as follows:

Let \( S = s_1 s_2 \dots s_n \) be the input string, and let \( L = [l_1, l_2, \dots, l_k] \) be the sequence of letters (i.e. \( l_i = s_j \) where \( s_j \) is a letter). Let \( L^R = [l_k, l_{k-1}, \dots, l_1] \) be the reversed sequence. Then, for each position \( j \) in \( S \), if \( s_j \) is a letter, replace it with the next element in \( L^R \); otherwise, keep it as is.

inputFormat

The input is provided via standard input (stdin) as a single line containing the string S. The string may contain alphabetic characters and digits.

outputFormat

Output the transformed string to standard output (stdout) after reversing only the letters in the string while keeping the digits in their original positions.

ab12cd34ef

fe12dc34ba