Apply Operations to Make String Empty

You are given a string s .

Consider performing the following operation until s becomes empty :

For every alphabet character from 'a' to 'z' , remove the first occurrence of that character in s (if it exists).

For example, let initially s = "aabcbbca" . We do the following operations:

Remove the underlined characters s = " a a bc bbca" . The resulting string is s = "abbca" .

Remove the underlined characters s = " ab b c a" . The resulting string is s = "ba" .

Remove the underlined characters s = " ba " . The resulting string is s = "" .

Return the value of the string s right before applying the last operation . In the example above, answer is "ba" .