Given a 0-indexed string s, repeatedly perform the following operation any number of times:

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Choose an index iin the string, and let cbe the character in position i. Delete the closest occurrence of cto the left of i(if any) and the closest occurrence of cto the right of i(if any).

Your task is to minimize the length of sby performing the above operation any number of times.

Return an integer denoting the length of the minimized string.

Example 1:

Input: s = "aaabc"Output: 3

Explanation: In this example, s is "aaabc". We can start by selecting the character 'a' at index 1. We then remove the closest 'a' to the left of index 1, which is at index 0, and the closest 'a' to the right of index 1, which is at index 2. After this operation, the string becomes "abc". Any further operation we perform on the string will leave it unchanged. Therefore, the length of the minimized string is 3.

Example 2:

Input: s = "cbbd"Output: 3

Explanation: 

For this we can start with character 'b' at index 1. There is no occurrence of 'b' to the left of index 1, but there is one to the right at index 2, so we delete the 'b' at index 2. The string becomes "cbd" and further operations will leave it unchanged. Hence, the minimized length is 3. 

Example 3:

Input: s = "dddaaa"Output: 2

Explanation: For this, we can start with the character 'd' at index 1. The closest occurrence of a 'd' to its left is at index 0, and the closest occurrence of a 'd' to its right is at index 2. We delete both index 0 and 2, so the string becomes "daaa". In the new string, we can select the character 'a' at index 2. The closest occurrence of an 'a' to its left is at index 1, and the closest occurrence of an 'a' to its right is at index 3. We delete both of them, and the string becomes "da". We cannot minimize this further, so the minimized length is 2.

用hashset更快；

Constraints:

1 <= s.length <= 100

scontains only lowercase English letters

Runtime: 5 ms, faster than 100.00% of Java online submissions for Minimize String Length.

Memory Usage: 43.8 MB, less than 25.00% of Java online submissions for Minimize String Length.