3563 - Lexicographically Smallest String After Adjacent Removals.

Hard

You are given a string s consisting of lowercase English letters.

You can perform the following operation any number of times (including zero):

• Remove any pair of adjacent characters in the string that are consecutive in the alphabet, in either order (e.g., 'a' and 'b', or 'b' and 'a').

• Shift the remaining characters to the left to fill the gap.

Return the lexicographically smallest string that can be obtained after performing the operations optimally.

Note: Consider the alphabet as circular, thus 'a' and 'z' are consecutive.

Example 1:

Input: s = "abc"

Output: "a"

Explanation:

• Remove "bc" from the string, leaving "a" as the remaining string.

• No further operations are possible. Thus, the lexicographically smallest string after all possible removals is "a".

Example 2:

Input: s = "bcda"

Output: ""

Explanation:

• Remove "cd" from the string, leaving "ba" as the remaining string.

• Remove "ba" from the string, leaving "" as the remaining string.

• No further operations are possible. Thus, the lexicographically smallest string after all possible removals is "".

Example 3:

Input: s = "zdce"

Output: "zdce"

Explanation:

• Remove "dc" from the string, leaving "ze" as the remaining string.

• No further operations are possible on "ze".

• However, since "zdce" is lexicographically smaller than "ze", the smallest string after all possible removals is "zdce".

Constraints:

• 1 <= s.length <= 250

• s consists only of lowercase English letters.