Smallest String With Each Letter Once

Remove letters from s so that every letter that appears in s appears exactly once, keeping the original relative order. Among all possible results return the one that is smallest in dictionary order.

For "bcabc" the answer is "abc"; for "cbacdcbc" it is "acdb".

Examples

Example 1

Input:

remove_duplicate_letters('bcabc')

Output:

'abc'

Example 2

Input:

remove_duplicate_letters('cbacdcbc')

Output:

'acdb'

Constraints

• 0 <= len(s) <= 200000
• Lower-case letters.